Six sandstone lithofacies (two from Location A, one from Location B, and three from Location C) were identified based on the dominant lithological characteristics which include texture, mineral composition, sorting, and sedimentary structure. These are (1) fine-grained sand injectite; (2) medium-grained planar cross-bedded sandstone; (3) coarse-grained thick-bedded sandstone; (4) medium-grained massive bedded sandstone; (5) conglomeratic sandstone and; (6) medium-grained trough cross-bedded sandstone.
Lithofacies 1 occurs as sand injectite or sand dyke within the marine shales of the Monterey Formation (Fig. 2a). Samples of this lithofacies are calcareous and consist of grey-brown fine to medium-grained sandstone with traces of very fine and coarse-size sand grains. The grains are triangular, lath-shaped, or irregularly polygonal with predominantly sharp and angular corners. Few of the grains, however, have rounded corners (Appendixes 1a, 1b). The sandstones are largely moderately well sorted (Fig. 3e, an average of 0.63 for sorting coefficient).
Figure 3. (a)–(d) Triangular plots used for classifying the sandstones; (e) bivariate plot of mean size vs. sorting meaning of lithofaciesa=a stratigraphic unit with a distinct lithologic feature or rock type.
Lithofacies 2 is the planar cross-bedded sandstone conformably underlying the marine shales of the Monterey Formation at Location A (Fig. 2a). Samples of this lithofacies are light-grey, calcareous and show traces of bioturbation. The sandstones are predominantly medium grained with traces of fine and coarse-size sand grains. The grains are lath-shaped or irregularly polygonal with angular to round corners. The grains are mostly well sorted (Figs. 3e, S1e, an average of 0.49 for sorting coefficient).
Lithofacies 3 unconformably overlies the crystalline basement rocks at Location B and consists of thick-bedded sandstones with massive to planar cross-bedding. This lithofacies is interbedded with lignite and siltstone beds and form an amalgamated multistorey braided channel-fill facies association (Fig. 2b). Samples of this lithofacies are grey to buff, non-calcareous, and consist of fine to very coarse-grained sandstones with pebbles. The grains are triangular, lath-shaped, or irregularly polygonal with predominantly sharp and angular edges. The pebbly grains show more rounded edges than the sand grains (Appendix 2a). The sandstones are mostly poorly sorted (Fig. 3e, an average of 1.0 for sorting coefficient).
Lithofacies 4 overlies the alluvial continental Elephant Trees Formation at Location C and consists of marine turbidite sandstone unit with mudstone and basalt, or megabreccia interbeds (Fig. 2c). Samples of this lithofacies are calcareous and consist of greyish-green very fine to medium-grained sandstones with the rare occurrence of coarse-size sand grains. Pebbles and siltstones are present only in trace amount. The grains are lath-shaped or irregularly polygonal with angular to round edges. Sorting varies from moderate to poor (Fig. 3e) with an average value of 0.92 for sorting coefficient.
Lithofacies 5 was sampled from the alluvial Elephant Trees Formation at Location C and consists of debris flow conglomeratic sandstones (Fig. 2c). Samples of this lithofacies are reddish-brown, fine to very coarse-grained sandstones with pebbles. The sand grains are triangular, lath-shaped, or irregularly polygonal with predominantly sharp edges. The grains are poorly sorted (Fig. 3e) with an average value of 1.10 for sorting coefficient.
Lithofacies 6 is the basal section underlying the alluvial Elephant Trees Formation at Location C (Fig. 2c) and consists of medium to coarse-grained planar to trough cross-bedded channel-fill sandstone. Samples of this lithofacies are reddish-brown, non-calcareous, and moderately sorted (average of 0.83 for sorting coefficient).
Lithofacies 1 is arkosic (Fig. 3d) with 28.6%–37.3% of quartz (average of 32.9%), 29.3%–31.1% of feldspar (average of 30.2%) and 2.8%–6.1% of lithic fragments (average of 4.5%). The quartz grains are mainly monocrystalline. K-feldspar (84% of the total feldspar content) is by far more abundant than plagioclase feldspar (16%, Fig. 4a) while rock fragments are mainly sedimentary with traces of igneous clasts. The grains are loosely packed with an average packing proximity index, PPI of 40% (Table 1) and grain boundaries dominated by point-contact type (Appendix 1a). Detrital matrix content is low (average of 1.0%). Microscopic observations indicate that this lithofacies is largely cement-supported (Appendixes 1a, 1b) with a total average cement of 29.3%. Calcite cement is the most common authigenic mineral (Fig. 4a, an average of 25.9%) and occurs as coarse mosaic pore-filling cement engulfing detrital grains, kaolinite clay cement and occluding intergranular pores (Appendixes 1b, 1c). Calcite cement here occurs as an early phase based on the loose grain packing or abundance of floating grains and absence of concave-convex grain contacts. Authigenic kaolinite (average of 1.0%) is the main clay cement observed in this lithofacies (Fig. 4a) and occurs as booklets of vermicularly stacked pseudo-hexagonal crystals (˃2 µm) filling intergranular pores (Appendix 1d) and replacing K-feldspars (Appendix 1b). Quartz cement occurs only as traces. Few of the detrital grains and calcite cement show evidence of microfracturing in the form of straight to curvo-linear traces (Appendix 1b) with poor dissolution. Both intergranular and secondary porosities are low (average of 3.4% and 0.3% respectively, Table 2).
Figure 4. Bulk X-ray diffractograms of representative powdered samples showing compositional variation among the identified lithofacies, (a)–(f). lithofacies 1–6 respectively. Kao. Kaolinite; Ano. anorthoclase (K-feldspar); Mcr. microcline (K-feldspar); Qtz. quartz; Calc. calcite; Mont. montmorillonite (smectite); Alb. albite (plagioclase feldspar); Oli. oligoclase (plagioclase feldspar).
Locations Lithofacies Sandstone type PPI (%) Grain boundary types (%) Grain/non-grain Point Long CC Suture A (San Joaquin) 1 Arkosic 40 34.9 14.3 1.6 0 49.2 2 Arkosic 46 19.3 38.7 11.2 0 30.8 B (Cajon Valley) 3 Arkosic 81 7.6 30.3 40.4 12.6 9.1 C (Salton Trough) 4 Lithic-arkosic 53 20.4 33.7 14.6 0 31.3 5 Lithic-arkosic 57 21.4 34.9 18.4 0 25.3 6 Lithic-arkosic 58 21.7 34.8 21.1 0 22.4
Table 1. The average packing proximity index (PPI) and grain boundary types for the identified lithofacies
Figure Appendix 1. (a) XPL photomicrograph showing early calcite cement and grain contact type; (b) BSE image showing early calcite cement, alteration of KF to kaolinite and weak compaction with point to long grain boundaries; (c) SEM image showing textural relationship between early calcite cement and authigenic kaolinite; (d) SEM image showing authigenic kaolinite with typical booklet habit; (e) PPL photomicrograph showing over view of the rock; (f) XPL photomicrograph showing textural relationship between early calcite and quartz cement; Quartz cement here encrusts the calcite cement; (g) SEM image showing weak compaction, albitization of feldspar and microfractures; (h) PPL photomicrograph showing secondary pore from feldspar dissolution. Calc1. Early calcite cement; Kao. authigenic kaolinite; P-contact. point contact type; L-contact. long contact type; Qo. quartz overgrowth; KF. K-feldspar grain; Qtz. quartz grain; PF. plagioclase feldspar grain.
Location Lithofacies Sandstone type Texture Composition(%) Cement(%) Thin-section porosity(%) IGV(%) IGP(%) MGS(φ) Sorting Qtz Fpar RF Matr Qtz Fpar Sm Ill-sm Kao Tot calc Pry por Sec por A(San Joaquin) 1 Arkosic 2.10 0.63 32.9 30.2 4.5 1.0 0.1 - - - 1.0 25.9 2.4 0.3 30.4 3.4 2 Arkosic 1.57 0.49 31.6 24.5 3.9 1.6 0.6 - - - 1.9 19.6 9.4 5.9 33.2 11.0 B(Cajon Valley) 3 Arkosic 0.98 1.01 22.1 44.3 4.2 5.9 5.6 1.4 0.4 2.9 - - 9.6 0.3 25.7 15.6 C(Salton Trough) 4 Lithic-arkosic 1.89 0.92 25.4 32.7 10.6 5.4 2.4 0.6 2.1 1.2 - 11.3 7.4 0.1 30.3 12.7 5 Lithic-arkosic 0.86 1.10 17.5 38.2 12.5 6.8 3.7 1.3 2.3 0.9 - 4.2 8.3 0.7 28.3 15.0 6 Lithic-arkosic 1.20 0.83 41.6 18.0 6.4 5.2 4.3 1.0 2.7 1.1 - - 12.3 0.8 27.1 17.6 Depo. Depositional, MGS. mean grain size; Qtz. detrital quartz; Fpar. detrital feldspar; RF. rock fragment; Matr. detrital matrix; Sm. Smectite; Sm-Ill. smectite-illite; Tot calc. total calcite; Pry por. primary porosity; Sec por. secondary porosity; IGV. intergranular volume; IGP. intergranular porosity.
Table 2. The average content of textural, compositional and thin section porosity characteristics of identified lithofacies from the study
Lithofacies 2 is arkosic (Fig. 3d) with 29.7%–37.6% of quartz (average of 31.6%), 23.9%–30.5% of feldspar (average of 24.5%), and 2.6%–6.7% of lithic fragments (average of 3.9%). The quartz grains are mainly monocrystalline. K-feldspar (64% of the total feldspar content) is more abundant than plagioclase feldspar (36%, Fig. 4b). Some of the feldspar grains show evidence of albitization with distinct irregular or zone-like texture in the feldspar grain (Appendix 1g). Lithic fragments consist mainly of igneous and metamorphic fragments. The grains are loose to moderately packed with an average PPI of 46% and grain boundaries dominated by point to long contact types (Appendix 1e). Detrital matrix content is low (average of 1.6%). Microscopic observations show that this lithofacies is largely cement-supported with a total average cement of 21.9%. Calcite cement is the most common (average of 19.6%) and occurs as a coarse mosaic pore filling phase engulfing detrital grains and kaolinite clay as in lithofacies 1 (Appendixes 1b, 1c). Kaolinite (average of 1.9%) occurring as pore-filling clay cement with booklets of vermicularly stacked pseudo-hexagonal crystals (˃2 µm) is the main clay cement observed in this lithofacies (Figs. 4b, 5a). At the same time, quartz cement occurs as traces (< 1.0%) as syntaxial overgrowth rimming round quartz grain and encrusting early calcite (Appendix 1f). Microfractured and dissolution pores are most pronounced in this lithofacies (Appendixes 1g, 1h). Average intergranular and secondary porosities are 11.0% and 5.9% respectively.
Figure 5. Clay fraction XRD patterns of representative powdered samples for (a) lithofacie B; (b) lithofacie C and (c) lithofacie F. Mont. montmorillonite (smectite); ill-Mont. illite-montmorillonite (smectite); Kao. kaolinite; Ano. anorthoclase (K-feldspar); Alb. albite (plagioclase feldspar).
Lithofacies 3 is arkosic (Fig. 3d) with 18.4%–27.6% of quartz (average of 22.1%), 39.5%–45.8% of feldspar (average of 44.3%), and 2.0%–6.3% of lithic fragments (average of 4.2%). The quartz grains largely consist of monocrystalline quartz. Plagioclase feldspar (58% of the total feldspar content) is more abundant than K-feldspar (42%, Fig. 4c). Some of the feldspar grains have albitized texture resembling lithofacies 2 (Appendix 1g). Lithic fragments are mainly sedimentary and metamorphic. The grains are tightly packed with an average PPI of 81% and grain boundaries dominated by long to concave-convex contact types with a minor proportion of point and sutured types (Appendix 2a, Table 1). The lithofacies has a high detrital matrix content (average of 5.9%). Microfractured grain with pseudo-matrix and highly deformed mica flakes also characterise this lithofacies (Appendix 2b). Microscopic observation indicates that this lithofacies is mostly grain-supported with a low total average cement of 9.8%. Quartz cement is the most common (average of 5.6%) and occurs as well developed euhedral syntaxial overgrowths partially rimming round quartz grain and lining intergranular pores (Appendix 2b). Authigenic smectite (average of 0.4%) characterises this sandstone unit and occurs as crenulated, box-like or webby pattern coating grains and lining pores. The pattern exhibited by this clay cement is like the texture recorded in Appendixes 2g, 3g–3h. Authigenic mixed-layer illite-smectite (montmorillonite) averages 2.9% in this lithofacies (Table 2). This clay cement occurs as web-like to fibrous texture with fine-hairy margin (Appendix 2c) with the web-like texture (smectite) being more pronounced than the fibrous-hairy habit (illite). Further examination of EDX spectrum and glycolated X-ray diffractogram (Appendixes 2d, 5b) confirm this clay as mixed-layer illite-smectite with the proportion of smectite (montmorillonite) to illite being about 60 : 40.
Figure Appendix 2. (a) XPL photomicrograph showing grain contact types; (b) XPL showing high compaction, grain contact types, quartz cement and deformed ductile grain; (c) SEM image showing illite clay cement with fibrous to lathlike texture; (d) EDX analysis showing illite clay cement; (e) BSE image showing late calcite cement in tight pore throat; (f) EDX analysis showing calcite cement; (g) SEM image showing textural relationship between late calcite, quartz and smectite cements. Here, Quartz overgrowth enveloped smectite and in turn, is engulfed by late calcite; (h) BSE image showing textural relationship between late calcite and feldspar cement. Here, late calcite engulfed feldspar cement. P-contact. Point contact type; L-contact. Long contact type; S-contact. Sutured contact type; Calc2. Late calcite cement; Sm. Smectite cement; Qo. Quartz overgrowth; Fo. feldspar overgrowth; KF. K-feldspar grain; Qtz.Quartz grain; PF. Plagioclase feldspar grain; Bio. Biotite grain.
Lithofacies 4 is lithic arkosic (Fig. 3d) with 23.7%–29.6% of quartz (average of 25.4%), 27.7%–40.1% of feldspar (average of 32.7%), and 5.9%–14.8% of lithic fragments (average of 10.6%). The quartz grains largely consist of monocrystalline quartz. Plagioclase feldspar (60% of the total feldspar content) is more abundant than K-feldspar (40%, Fig. 4d). Some of the feldspar grains have albitized texture resembling that of lithofacies 2 and 3 (Appendix 1g). Lithic fragments are mainly granitic and metamorphic. Samples of this lithofacies show variation in grain packing from moderate to tight with an average PPI of 53% and grain boundaries ranging from point to concave-convex contact types. Grain sutured contact is absent. Detrital matrix ranges from 2.3% to 7.5% (average of 5.4%). Mica flakes are mostly biotite and muscovite grains and show moderate to high plastic deformation. This lithofacies is grain to cement-supported with a total average cement of 15.8%. Calcite cement is the most common (average of 11.3%) and occurs as two generations (early and late) based on textural relationship (Appendix 4). The early calcite phase occurs as coarse mosaic cement filling loosely packed sandstone with free pore throat and engulfed detrital grain and authigenic smectite as in lithofacies 1 and 2 (Appendixes 1c, 1d, 4). The late calcite phase occurs as fine mosaic pore-filling cement that is precipitated into relatively denser packed sandstone with tight pore throat and engulfed authigenic quartz and feldspar (Appendixes 2e–2h). Statistical analysis from mapping these two calcite forms from SEM images shows that the late calcite cement is by far more pronounced (average of 71% of the total calcite) in this rock unit than the early calcite cement (average of 29%). Quartz cement (average of 2.4%) also developed in two forms (macro quartz and micro quartz). The macro quartz (average thickness exceeding 10 µm) is the dominant type and occurs as partial to well developed, euhedral syntaxial overgrowths rimming around detrital quartz grain and lining intergranular pores (Appendixes 2e, 2g). The micro quartz cement (average thickness of less than 10 µm) occurs as small euhedral to anhedral isolated to clustered quartz crystals in primary pores (Appendix 4a). Authigenic feldspar, smectite and illite-smectite are present in varied proportion (Table 2). Average intergranular porosity is 12.7% while secondary porosity occurs as traces.
Figure Appendix 3. (a) XPL photomicrograph showing late calcite cement in pore; (b) SEM showing late calcite cement in tight pore throat; (c) XPL photomicrograph showing late calcite cement filling microfractured and dissolution pore; (d) XPL showing textural relationship between late calcite, quartz overgrowth and microfracture; (e) XPL photomicrograph showing feldspar overgrowth; (f) XPL photomicrograph showing feldspar overgrowth flowing into pore and encrusting smectite; (g) SEM image showing authigenic smectite with web-like or crenulated texture; (h) EDX showing authigenic smectite. Calc2. Late calcite cement; Sm. smectite cement; Qo. quartz overgrowth; Fo. feldspar overgrowth; Fog. feldspar outgrowth; KF. K-feldspar grain; Qtz. quartz grain; PF. plagioclase feldspar grain; Ep. epidote grain; M. muscovite grain.
Figure Appendix 4. (a) EDX-assisted interpreted SEM image showing textural relationship between smectite, microcrystalline quartz, early calcite and late calcite cements. Here, early calcite envelops smectite, microcrystalline quartz encrusts smectite, while late calcite engulfs all three cements; (b) Uninterpreted SEM image. Calc1. Early calcite cement, Calc2. late calcite cement, McQ. microcrystalline quartz cement.
Lithofacies 5 is lithic arkosic (Fig. 3d) with 14.2%–23.9% of quartz (average of 17.5%), 31.9%–42.5% of feldspar (average of 38.2%), and 11.6%–13.3% of lithic fragments (average of 12.5%). The quartz grains are dominantly monocrystalline quartz. Plagioclase feldspar (68% of the total feldspar content) is more abundant than K-feldspar (32%, Fig. 4e). Some of the feldspar grains show albitized texture. Lithic fragments are mainly granitic and metamorphic. The grains are moderate to tightly packed with an average PPI of 57% and grain boundaries dominated by point to concave-convex type. Detrital matrix ranges from 5.3% to 8.1% (average of 6.8%). Mica flakes are mostly biotite and muscovite grains and show moderate to high plastic deformation. This lithofacies is grain to cement-supported with a total average cement of 13.3%. Calcite cement (average value of 4.2%) is slightly more abundant than authigenic smectite (average of 2.9%), illite-smectite (average of 2.3%), quartz (average of 3.7%), and feldspar (average of 1.3%). Textural relationship reveals that the calcite cement occurs as a late phase filling tight pore throat, engulfed authigenic quartz and feldspar, and healed up microfractured and dissolution pores (Appendixes 3a–3d). Average intergranular porosity is 15.0% while secondary porosity occurs as traces due to the infilling of late calcite cement.
Lithofacies 6 is lithic arkosic (Fig. 3d) with 35.1%–47.4% of quartz (average of 41.6%), 16.1%–20.9% of feldspar (average of 18.0%), and 4.7%–8.0% of lithic fragments (average of 6.4%). The quartz grains are dominantly monocrystalline quartz. Plagioclase feldspar (59% of the total feldspar content) is more abundant than K-feldspar (41%). Some of the feldspar grains show traces of albitized texture resembling that of lithofacies 2 (Appendix 1g). Lithic fragments are mainly granitic and metamorphic. The grains are moderate to tightly packed with an average PPI of 58% with grain boundaries dominated by point to concave-convex type. Detrital matrix averages 5.2%. Mica flakes show slight to high deformation. This lithofacies is grain to cement-supported with a total average cement of 9.5%. Quartz cement is the most common (average of 4.3%) and occurs as syntaxial overgrowths rimming around detrital quartz grain and lining intergranular pores. Feldspar cement is in varied amount (average of 1.3%) and occurs as both overgrowth lining pores and outgrowth filling intergranular pores (Appendixes 3e, 3f). Authigenic smectite (Appendixes 3g, 3h) and illite-smectite (Fig. 5c) also occurs in different amount. Microfractured and partially dissolved detrital grains also characterise this lithofacies. The average values for intergranular and secondary porosities are 17.6% and 0.8% respectively.
3.1. Lithofacies Description of Sandstones from Hand-Specimen
3.2. Petrographic Description from Thin Sections
Reservoir petrofacies is the classification of rocks based on depositional parameters (e.g., structures, textures, and primary composition) and diagenetic fabrics which control the porosity and permeability of the rock (Jardim et al., 2011; De Ros and Goldberg, 2007). This study has identified four possible types of petrofacies (Fig. 6b) based on the rock depositional parameter, diagenetic signatures and their visual porosity estimated values.
Figure 6. Plot of (a) intergranular volume (IGV) vs. cement volume after Houseknecht (1987); (b) compaction porosity loss (COPL) vs. cementation porosity loss (CEPL) after Lundegard (1992) showing four distinct clusters (P1–P4) indicating four preferential mechanisms of porosity alterations in the reservoir petrofacies. P1. petrofacies 1; P2. petrofacies 2; P3. petrofacies 3; P4. petrofacies 4.
Petrofacies P1 is present mainly in the sand injectite or sand dyke within the Monterey Formation in the San Joaquin forearc basin (Fig. 2a). These sandstones are predominantly fine-grained, moderately well sorted, arkosic, weakly compacted but highly cemented with sparse dissolution pores (Tables 3, 4; Figs. 7b–7f, 9). The high textural properties of these sandstones could have translated into a high reservoir quality due to the relatively low amount of detrital and labile components that usually clog pores and promote rapid compaction (Ketzer and Morad, 2006; Morad et al., 2000).
Intensity COPL (%) CEPL(%) Pdissolution (%) Strong > 20 > 20 > 70 Medium 20–10 20–10 70–30 Weak 10–0 10–0 30–0
Table 3. The classification standard of influence extent on reservoir quality
Petrofacies Lithofacies Sorting Total ductile
Mineral composition COPL(ave., %) CEPL(ave., %) Pdissolution(ave., %) Intergranular porosity Pore filling cement P1 1 Moderate well
1.8–2.1(1.9) Arkosic 8.0–10.7(9.4) 25.9–27.3(26.6) 8.5–11.3(9.9) 2.8–4.1(3.4) Calcite(ave.25.9%); Quartz(ave.0.2%);
kaolinite(ave.1.0%); Smectite(?); Illite(?)
P2 2 Well sorted 2.2–2.4(2.3) Arkosic 6.3–10.9(8.8) 18.7–22.3(20.0) 35.3–41.8(38.4) 10.1–12.0(11.0) Calc(ave.19.6%); Qtz(ave.0.6%);
Kaolinite(ave.1.9%); Smectite(?); Illite(?)
P3 4, 5 Moderate to poor 3.2–9.4(7.5) Lithic arkosic 1.3–12.0(9.3) 9.9–25.5(15.5) 0.0–7.7(1.6) 9.2–15.6(12.5) Calcite(ave.10.3%); Qtz(ave.3.1%); Kaolinite(?);
P4 3, 5, 6 Moderate to poor 5.5–12.6(8.7) Lithic arkosic 7.8–18.5(14.4) 4.5–9.3(8.1) 0.0–9.8(5.6) 12.6–18.9(16.7) Calcite(ave.0.1%); Qtz(ave.4.6%);
Kaolinite(?); Smectite(ave.1.8%); Illite-smectite(ave.1.6%)
COPL. Compaction porosity loss; CEPL. cementation porosity loss; Pdissolution. dissolution porosity
Table 4. The textural, compositional, diagenetic and porosity characteristics of the petrofacies types in the study
Figure 7. Plots of Intergranular porosity vs. (a) mean grain size; (b) intragranular porosity; (c) calcite cement; (d) quartz cement; (e) total clay cement; (f) total cement content.
Figure 9. Paragenetic sequence of the main diagenetic features in the Miocene sandstones from the study area.
However, the pervasive pore-filling early calcite cement significantly obliterated intergranular pores of these sandstones and developed extremely isolated pores with values ranging between 2.8% to 4.1% (average of 3.4%, Table 4). Because isolated pores have poor interconnectivity resulting in low permeability values (Worden and Matray, 1998) as seen in this rock unit, P1 is therefore considered to have poor reservoir quality.
Petrofacies P2 occurs mainly in the submarine fan sandstones underlying the Monterey Formation in the San Joaquin forearc basin (Fig. 2a). The sandstones are medium-grained, well sorted, arkosic, weakly compacted, moderately cemented with medium dissolution pores (Tables 3, 4, Figs. 7b–7f, 9). Like P1, the high textural properties of these sandstones should have also translated into high reservoir quality but for the pervasive pore-filling early calcite cement, although lower in volume than in P1. Intergranular porosity of these sandstone lies between 10.1% and 12.0% with an average value of 11.0%. However, unlike P1, these sandstones show evidence of medium dissolution of feldspar grains with a resultant effect of large moldic pores which are likely to promote pore connectivity (Jiang et al., 2010; Schmidt and Mcdonald, 1984). Hence, permeability values in these sandstones are expected to be higher than in P1.
Petrofacies P3 is present mainly in the marine turbidite and marine-influenced distal alluvial fan sandstones in the Salton Trough strike-slip basin. These sandstones are medium to coarse grained, moderately sorted, arkosic to lithic arkosic, weakly compacted, moderately cemented with weak feldspar dissolution. In comparison to P1 and P2, these sandstones have slightly higher intergranular porosity values in the range of 9.2%–15.6% (average 12.5%). The relatively higher intergranular porosity of these sandstones despite having relatively lower textural properties is related to the lower volume of both early and late calcite cement filling pores. Secondary porosity in these sandstones is mainly in the form of microfractured pores formed due to mechanical compaction (Zeng, 2010). The infilling of both the microfractured and intergranular pores by late calcite cement would significantly affect their interconnectivity, hence have a great impact on their permeability. Permeability values are therefore expected to be lower than those of P2. Furthermore, subsequent diagenesis of illite-smectite clay will lead to the authigenesis of illite in this sandstone (Worden and Morad, 2003). Because illite tends to form a pore-bridging textural pattern, their presence in these rocks is likely to further affect their permeability values (Morad et al., 2012).
Petrofacies P4 is present mainly in the continental sandstones in both Cajon Valley and Salton Trough strike-slip basins. These sandstones have the least influence of marine incursion in the study as evidenced by the near absence of early calcite cement (Table 4). The sandstones are medium to very coarse-grained, pebbly, moderately to poorly sorted, arkosic to lithic-arkosic, moderately compacted, weakly cemented with infrequent dissolution pores. Although these sandstones have the worst textural properties, their intergranular porosity values (12.6%–18.9%, average 16.7%, Table 4) are the highest in the study and is interpreted to result from the near absence of early calcite cement. However, unlike the three previous petrofacies, these sandstones are the most affected by mechanical compaction (Fig. 6) as evidenced by the presence of sutured grain contacts and pseudomatrix (Appendix 2b), and this is attributed to the presence of large amount of labile components which promoted rapid mechanical compaction (Ketzer and Morad, 2006; Morad et al., 2000). The presence of pseudomatrix clogging pores, together with the relatively high amount of authigenic illite-smectite is likely to affect their flow properties.
Depositional environments control the character of their sandstone deposits such as sand body architecture, sand-to-mud ratio, textural properties (e.g., grain size, sorting), primary porosity and permeability, pore water chemistry and early diagenetic alterations (Lan et al., 2016; Morad et al., 2012). In this study, P1, P2 and P3 are marine occurring as submarine fans to turbidite sandstones whereas P4 occurs as continental alluvial fans to braided sandstones (Figs. 2, 8). The marine sandstones (P1–P3) have higher textural properties with moderate to well sorting, whereas the continental sandstones (P4) have lower textural properties with poor sorting. The high textural properties among P1–P3 in comparison to P4 can be attributed to the transport distance from the source area and the paleo-flow energy characterizing their deposition. Because the transport distance between the source area and these depositional settings is quite long, hydraulic sorting, abrasion, marine wave and storm processes combine to rework these sediments more efficiently to yield the observed textural characteristics.
Figure 8. Paleogeography diagram of southern California area during deposition of Miocene sand deposits (modified from Parrish, 2006). (a) Submarine fan sand deposits overlain by deep marine fines during progressive transgression in the fore-arc San Joaquin Basin; (b) braided channel sand deposits formed by regression in the Cajon Valley strike-slip basin; (c) continental sand deposits punctuated by relatively short marine transgression followed by a regressive phase in the Salton Trough strike-slip basin.
On the other hand, the low textural attributes of P4 is attributed to the relatively short transport distance between the source and the depositional setting. Hence, the effect of hydraulic sorting and abrasion action on these sediments is relatively lower. Besides, the absence of waves and storms which are considered as effective reworking processes (Boggs, 2006) are also absent in these continental environments, thus causing these sediments to retail high matrix content. The depositional environment also controls the early diagenesis of these sandstones by dictating the chemistry of pore water as further explained in Section 4.3.
The major diagenetic processes controlling the petrophysical properties of the Miocene sandstones in the study include mechanical compaction, cementation, dissolution, microfracturing, and mineral replacement. The paragenetic sequence, which is based primarily on the distribution pattern of the diagenetic minerals and their textural relationships as determined from thin sections and SEM-BSE images, are shown in Fig. 9. It generally includes two stages of evolution based on the definition of Oluwadebi et al. (2018).
Early diagenesis refers to processes that affect sediments after deposition due to their proximity to the surface (Oluwadebi et al., 2018 and reference therein). This diagenetic regime occurs at shallow burial depths with a maximum burial depth of 1–2 km and low-temperature range of 30–70 ℃ (Worden and Burley, 2003; Morad et al., 2000). During this stage of diagenetic evolution, the chemistry of the interstitial waters is controlled mainly by the depositional environments (Schmid et al., 2004).
The textural relationship between authigenic clays (kaolinite and smectite) and early calcite (Appendixes 1c, 1d, 4a) indicates that these clay cements were formed at the early stage of diagenesis. Petrographic investigation, however, indicates that while kaolinite cement is present in petrofacies P1 and P2 (Location A-San Joaquin Valley), authigenic smectite is found in petrofacies P3 and P4 (Cajon Valley and Salton Trough). The distribution of these early clays in the study suggests the influence of paleoclimatic conditions on the position and chemistry of the interstitial waters which dictated their formation (Lai et al., 2015; Schmid et al., 2004). The presence of kaolinite clays in petrofacies P1, and P2 is suggestive of sediment deposition in a climatic condition with net rain fall. The influx of this water cause pore water to become acidic with relatively high H+/K+. The interaction of this acidified water with rocks lead to the leaching and kaolinization of unstable silicate grains, as observed in Appendix 1b (Lai et al., 2015; Bjørlykke, 2014; Ketzer et al., 2003; Worden and Morad, 2003). The fact that the leaching effect was mainly on the feldspar grain but not on the early calcite cement provides a further attestation that this diagenetic phase preceded early calcite cementation.
On the other hand, the presence of authigenic smectite in petrofacies P3 and P4 is indicative of deposition under a paleoclimatic condition with net evaporation. Under this condition, meteoric influx resulting in ionic leaching is limited. Hence, pore waters become hypersaline with a high Mg2+/H+, and K+/H+ ratios leading to the precipitation of this cement (Bjørlykke, 2014; Mckinley et al., 2003; Worden and Morad, 2003).
The most important early diagenetic event is the pervasive precipitation of early calcite cement across the three basins which strongly influence the petrophysical properties of these sandstones (Fig. 7c, Table 4). This calcite cement occurs prior to mechanical compaction as indicated by the loose grain packing, and microfracturing of this cement with detrital grains (Appendixes 1c, 1g). Early formed calcite cement in sandstones is often defined by the presence of loose grain packing (Hakimi et al., 2012). The early calcite cementation prevented further diagenetic modifications by mechanical compaction in these samples except for a later stage of detrital grain dissolution and microfracturing. The high calcite cement in the marine sandstones is likely due to the high concentration of Ca2+-cation that normally characterises marine water (Hakimi et al., 2012; Morad, 1998). Point count data shows that this early calcite cement is by far most pronounced in P1 and P2 from the San Joaquin Basin, it is reduced in P3 in the Salton Trough while it is highly insignificant in the continental P4 sandstones. The high prevalence of early calcite cement in P1 and P2 in the San Joaquin Basin is attributed to the relatively more prolonged exposure of these sandstones to Ca2+-rich marine water during the progressive deepening of the basin as evidenced by the thick marine succession (Figs. 2a, 8a).
Conversely, the lower amount of early calcite cement in P3 is interpreted to result from the increasing influx of clastic sediments from the Colorado rivers during the Late Miocene base-level fall (Dorsey et al., 2011) as evidenced by the thickening-upward sequence (Figs. 2c, 8c). The poor early calcite cementation in the continental P4 sediments is interpreted to result from the limited exposure of these sandstones to Ca2+-rich marine water. Textural relationships from petrographic investigation show that this early calcite cement was engulfed by quartz overgrowth (Appendix 1f) and therefore indicate that this cement was precipitated at a temperature lower than the typical temperature (˃70 ℃) required for quartz precipitation (Bjørlykke and Jahren, 2010; Marcussen et al., 2010; Worden and Morad, 2003).
As effective stress resulting from burial increased with depth, early mechanical compaction squeezed the grains together, reducing the inter-grain pore distance and resulting in the transformation of grain boundaries from the floating type at the onset of burial to predominantly point contact with long type (Appendixes 1a–1c). The evolution of grain boundaries with depth reduces the intergranular porosity of sandstones (Lai et al., 2016; Hakimi et al., 2012).
Late diagenesis refers to processes affecting rocks that are isolated from surface-related processes (Schmid et al., 2004) with burial depths exceeding 2 km and temperatures above 70 ℃ (Morad et al., 2000). This diagenetic phase is controlled by the rock-water interactions, composition and distribution of earlier-formed authigenic minerals (Mansurbeg et al., 2008; Morad et al., 2000).
Under this diagenetic regime, progressive burial and mechanical compaction of the Miocene sandstones continued to reduce the primary intergranular porosity as detrital grains were further squeezed together. Mechanical compaction at this stage produced more compacted grain boundaries characterised dominantly by long, concavo-convex and stylolitic grain contacts (Appendix 2a). Other evidence of this compaction includes plastic deformation of ductile grains, the formation of pseudo-matrix, and microfracturing of both detrital grains and early calcite cement (Appendixes 1g, 2b). The development of microfracturing during diagenesis has previously been related to stresses resulting from the shrinkage of rock constituents or whole rocks by mechanical compaction (e.g., Zeng, 2010; Schmidt and McDonald, 1984). Across the study area, mechanical compaction had the most significant impact on petrofacies P4 as revealed by its average highest COPL value (Table 4). The high mechanical compaction recorded by this rock unit is attributed to a variety of factors which include the near absence of early calcite cement and the presence of high ductile components. Early calcite cement in sandstones stabilises the rock framework and prevent mechanical compaction (Morad et al., 2019; Molenaar, 1990). Sandstones enriched in ductile grains have been reported to experience high rate of mechanical compaction with a progressive depth of burial leading to rapid loss of porosity and permeability in contrast to sandstones enriched in rigid grains (Ketzer and Morad, 2006; Smosna and Bruner, 1997).
At progressive burial temperature, authigenic smectite transformed to mixed smectite-illite clay with increasing illite component as burial depth and temperatures increase (Wilkinson et al., 2006; Środoń, 1999; Pusch and Kanland, 1996; Velde, 1986). The transformation of smectite to illite through mixed smectite-illite commonly occurs at depths usually exceeding 2 km or at a temperature above 70 ℃ (Bjørlykke, 2014; Worden and Morad, 2003). The presence of mixed smectite-illite in petrofacies P3 and P4 with higher smectite component, therefore, marks a transition from an early diagenetic phase to a late diagenetic realm with much of these sandstones buried at the onset of the late diagenetic phase. There was no illite in P1 and P2 and therefore suggest that the temperature regime (˃70 ℃) required for the transformation of kaolinite to illite has not been attained in the basin despite the significant presence of K-feldspar in these rocks (Bjørlykke, 2014; Worden and Morad, 2003).
Quartz and feldspar authigenesis in the study is attributed to chemical compaction influenced by increased burial stress and temperature. Evidence of chemical compaction in the study comes from the presence of microstylolites or grain sutured boundary (Sheldon et al., 2003; Worden and Morad, 2000) as observed in Appendixes 2a, 2b, 3e, 3f. Although it was difficult to determine the relative timing between these two authigenic minerals, the textural relationship however shows that these authigenic minerals engulfed early formed clay cement but are enveloped by late calcite cement (Appendixes 2e, 2g–2h, 3d, 4a). Previous studies (e.g., Bjørlykke and Jahren, 2010; Marcussen et al., 2010; Schmid et al., 2004; Worden and Morad, 2003; Harper et al., 1995) have considered authigenic feldspar and quartz as late diagenetic cements commonly developed above 70 ℃. Quantitative analysis of point count data shows that the volume of authigenic quartz cement is directly related to the COPL values (Table 4) thus further confirming that most of the silica cement in the rocks were derived from chemical compaction. Although the transformation of smectite to illite has been reported to also contribute to silica authigenesis (Worden and Morad, 2003), the contribution of this to the overall silica cement in the study is likely to be low considering the relative minute volume of authigenic smectite and illite-smectite clays recorded by the rock.
Another late diagenetic phase recorded in the study is the albitization of feldspar based on the textural relationship observed in Appendix 1g. Albitized texture is formed by the combination of stress resulting from compaction and burial temperature (Ramseyer et al., 1992).
Late dissolution of both early calcite cement and detrital feldspar grains is most pronounced in petrofacies P2 (Appendix 1h) and is interpreted here to result from the interaction of acidic fluids with the rock. Two main sources of acidic fluids are responsible for the dissolution of rocks in the literature. The first source is related to the influx of meteoric waters driven by fault-related topography (Wilkinson et al., 2006) while the second source result from the migration of organic acids and CO2 generated by the thermal maturation of organic matter in source-rocks (Mansurbeg et al., 2008). Considering that this dissolution occurs as a late diagenetic event (Fig. 9) combined with the apparent disparity of the effect of this dissolution on P2 in comparison with the other Petrofacies, it is unlikely that this dissolution may have been caused by meteoric influx which is often considered as an early diagenetic process (Mansurbeg et al., 2008). This late dissolution which have resulted in large secondary-moldic pores in P2 is believed to have been caused by organic acids and CO2 generated by the thermal maturation of organic matter in the source-rock marine shales of the underlying Kreyanhagen Formation. It could also have been formed from the adjacent Monterey shale separated by a fault which perhaps acted as a conduit for the migration of the fluids (Figs. 2a, 8a). The poor dissolution of P1 despite being present in the same basin as P2 may be due to its limited exposure to these acidified fluids. Evidence of organic acid-and CO2 generated source rocks are lacking in the other two basins, hence do not affect the sandstone deposits.
Post-compaction calcite cementation (Calc 2) marks the last phase of this late diagenetic event in the study (Fig. 9). Petrographic observation, together with point count data shows that this calcite cement is most significant in petrofacies P3 sandstone filling intragranular and moldic pores (Appendixes 3c, 4). Petrofacies P1 and P2 show no evidence of this cement while P4 only recorded traces. Late diagenetic calcite has been observed to fill small pores between tightly packed framework grains, indicating precipitation after considerable compaction (Lai et al., 2016). The formation of late calcite cement in this study may have been associated with the albitization of Ca-plagioclase (Schulz et al., 1989).
The reservoir quality variation in the region was controlled to some extent by depositional processes which dictated the grain size, depositional matrix and sorting characteristic of the rocks (Fig. 7a). However, the most significant controls are by diagenetic alterations which largely dictated the distribution of the petrofacies types and therefore have an enormous implication on the reservoir quality variation in the region (Figs. 7, 9). The most considerable diagenetic alteration affecting the reservoir quality of these sandstones is calcite cementation. This cement, together with the dissolution of unstable grains and compaction are the dominant factors which have dictated the classification of the identified lithofacies into their petrofacies types (Figs. 6b).
Evidence from this study shows that early calcite cementation is most extensive in the San Joaquin forearc basin and had the most significant control on the reservoir quality of petrofacies 1 and 2 (Fig. 7c). However, late dissolution by acidic pore fluids had the most significant effect on petrofacies 2 (submarine fan sandstones) perhaps, because of its lateral exposure than petrofacies 1 (sand injectite or dyke). The developed large moldic pores in petrofacies 2 (Appendix 1h) are likely to increase total porosity and permeability values of these rocks (Fig. 7b). Additionally, the absence of illite clay cement and a relatively low concentration of authigenic quartz (Table 4) combine to make these sandstones the best reservoir target in this basin. However, the pervasive calcite cementation of petrofacies 1 (sand injectite or dyke), insufficient dissolution by acidic pore fluids, coupled with their limited area would combine to make this petrofacies a less reservoir target relative to petrofacies 2 in the basin.
In the strike-slip basins (Cajon Valley and Salton Trough), the high intergranular porosity combines with the near absence of early calcite cement in petrofacies P4 (continental sandstones) makes these rocks the best reservoir target in this basin. However, because these sandstones are also characterised by a relatively large quantity of labile components with illite-smectite cement, their permeability values are expected to be relatively lower.
Comparing the possible rate of diagenesis between the two basins, the strike-slip basins (Cajon Valley and Salton Trough) have higher thermal characteristics ranging from about 60 mWm-2 to over 150 mWm-2 (Ben-Avraham et al., 2010; Sass et al., 1994). On the other hand, the San Joaquin forearc basin has lower thermal characteristics (> ~20–40 mWm-2, Tian and Liu, 2013; Lutz et al., 2011). Because of this variation in thermal characteristics between the two basins, the diagenesis of petrofacies P4 (continental sandstones) in the strike-slip basins is expected to proceed much rapidly over time leading to abrupt deterioration of reservoir quality in comparison to that of petrofacies 1 (sand injectite or dyke) and petrofacies 2 (submarine fan sandstones) in the San Joaquin forearc basin. Hence, the best bet for exploration and reservoir targets in the southern California region are the sandstones (petrofacies 1 and 2) in the San Joaquin Basin.
The studied petrofacies 1, and 2 in the San Joaquin forearc basin are respectively analogous to the petroliferous Miocene injectite complex (Yellow Bank Creek Complex, Santa Margarita sandstone, Thompson et al., 2007) and Stevens sandstone reservoirs (Mahon et al., 1998; Boles and Ramseyer, 1987). Emplacement of hydrocarbon in these sandstone reservoirs post-dated early calcite cementation and filled late dissolution pores. Although these secondary pores increase the pore network connectivity to some extent, most of these liquid hydrocarbon accumulations, particularly in the Steven sandstones reservoirs, are still largely compartmentalised by early calcite cement (Mahon et al., 1998). Hence, for maximum recovery of the accumulated liquid hydrocarbon in these sandstone reservoirs, the use of acidified fluids would help to dissolve the calcite-filled pores, increase their pore network connectivity and enhance flow potential of these reservoirs.