The newly-obtained zircon U-Pb ages show that Jurassic magmatic rocks did not develop in the Huanghua depression, and the Cretaceous igneous rocks could be divided into two sets, namely, the Early Cretaceous and the Late Cretaceous (Table 1). Early Cretaceous igneous rocks are widely distributed in the north zone, with zircon U-Pb ages varying from 140 to about 112 Ma, lasted for about 30 Ma. The Late Cretaceous igneous rocks that was distributed in the south zone lasted from 75 to 70 Ma. In addition, Mesozoic granite only developed in the late stage and distributed sporadically in the Yanshan area to the east of the Wangguantun area. In summary, the early phase mainly developed intermediate-basic rocks, while the late phase developed intermediate-acid rocks.
Area Well No. Depth (m) Lithology Age (Ma) Era Data source Yanshan Y1 2 172 Granite 74.0±2.1 K2 * Zaoyuan Z1532 2 778 Andesite 72.0±1.9 K2 * Zaoyuan Z55 2 994 Andesite 75.77±2.02 K2 ** Zaoyuan F22-15 2 950 Dacite-porphyrite 71.5±2.6 K2 Zhang et al. (2011) Zaoyuan F22-15 2 944 Dacite-porphyrite 69.95±0.78 K2 Zhu et al. (2019) Zaoyuan F22-15 2 956 Dacite-porphyrite 74.89±0.96 K2 Zhu et al. (2019) Yangsanmu Y23 1 525 Tuffite 111.46±0.76 K1 Zhu et al. (2019) South Dagang Qg101 3 198 Basalt 114.1±1.4 K1 This study South Dagang Qg101 3 150 Andesite 115.6±4.7 K1 This study South Dagang Qg8 3 373 Andesitie Porphyrite 116.0±1.4 K1 This study South Dagang Qg1601 2 776 Tuffaceous sandstone 117.7±1.5 K1 Zhang F P et al. (2019) Koucun K23 Basalt 114.8±2.8 K1 Guo et al. (2012) Koucun K36 1 695 Andesite 118.8±1.0 K1 Zhang et al. (2011) Koucun K36 1 685 Andesite 123.4±2.2 K1 Zhang F P et al. (2019) Xianzhuang X6 1 776 Andesite 122.2±1.09 K1 Zhu et al. (2019) Xianzhuang X6 2 171 Volcanic breccia 123.1±1.09 K1 Zhu et al. (2019) South Dagang Qg2 2 325 Basalt 140.1±1.4 K1 This study South Dagang Qg2 2 996 Basalt 138.7±4.06 K1 Gao and Zhang (1995) South Dagang Qg2 Basalt 133±20 K1 Guo et al. (2012) *. Unpublished data of Changqian Ma; **. unpublished data of Dagang Oilfield Company.
Table 1. Ages of Cretaceous igneous rocks in the Huanghua depression
Based on zircon U-Pb data, lithology of the igneous rocks and sedimentary layers, Zhu et al. (2019) subdivided the Early Cretaceous magmatism into two cycles. The early cycle formed between 125 and 120 Ma, while the late one lasted from 110 to 100 Ma. Moreover, an earlier cycle had been denuded before the 125–120 Ma cycle has been identified (Zhu et al., 2019). Combined with the newly-obtained data, we can divide the Early Cretaceous magmatism from the Huanghua depression into 3 cycles, in which cycle Ⅰ happened in 140 Ma, cycle Ⅱ between 125 and 119 Ma, and cycle Ⅲ between 118 and 111 Ma (Fig. 7). The erosion of the Upper Cretaceous strata was recorded in all the wells, which is consistent with the previous conclusion that the Huanghua depression had experienced uplift and denudation in the Late Cretaceous (Wu et al., 2020).
Figure 7. Lateral comparison chart of the Early Cretaceous igneous rocks in the Huanghua depression. The chart agrees with line 1 in Fig. 1, data sources correspond to Table 1.Well Qg2 represents the characteristics of cycle Ⅰ, wells K36 and X6 represent the characteristics of cycle Ⅱ, wells Qg101 and Qg8 represent the characteristics of cycle Ⅲ, respectively. SP. Spontaneous potential logging; GR. natural gamma ray logging; Mz. Mesozoic; P. Permian; C. Carboniferous; Es1, Es3. the first and the third members of Eocene Shahejie Formation; Ed. the Eocene Dongying Formation.
Cycle Ⅰ started at 140 Ma, with typical lithology of basalt that was only discovered in the North Dagang area. Guo et al. (2012) conducted zircon U-Pb dating on basalt sample from Well Qg2 and obtained the age of 133±20 Ma. This age was calculated only based on four zircon grains and resulted in a large error of 20 Ma. Our newly-obtained zircon U-Pb age of 140.1±1.4 Ma from basalt sample of 2 325 m depth is identical the age 139 Ma obtained from Gao and Zhang (1995) on basalt at the depth of 2 996 m. This implies that the two volcanic layers formed in a short period with about 700 m of sedimentary interval between them. Since there was no fault had been discovered in the thorough-Well Qg2 seismic section, we supposed the "basalt" sample tested by Gao and Zhang (1995) was either subvolcanic rock or hypabyssal intrusive rock such as basaltic porphyrite or diabase. Since the rock debris used in lithology naming were difficult to identify.
Cycle Ⅱ was formed between 125 and 119 Ma and developed in the north zone, with dominant lithology of basalt, andesite, and volcanoclastic rocks. This cycle Ⅰs mainly distributed in the Yangsanmu and Koucun areas. The typical wells X6 and K36 in Koucun represented the bottom-center and center-top parts of this cycle. The Well X6 developed andesite, basalt, and volcanoclastic rocks and tuff in the depths of 2 170–2 089, 2 039–2 016, and 1 796–1 778 m, respectively. The Well K36 developed interbedding of basalt and mudstone, volcanoclastic rocks and tuff, and andesite in the depths of 1 880–1 844, 1 784–1 826 and 1 666–1 760 m, respectively. The volcanoclastic breccia of Well X6 was dated at 122.2±1.09 Ma by Zhu et al. (2019), and the andesite of Well K36 was dated at 123.4±2.2 to 118.8±1.0 Ma by Zhang F P et al. (2019) and Zhang et al. (2011). This indicates these rocks were formed in a very close time. The stratigraphic sequences of X6 and K36 show that medium-thick andesite and basalt layers are followed by thick layers of volcanoclastic rocks and tuff, and thick layers of andesite, it is indicated that the magma activity was a relatively quiet eruption and occasionally accompanied by an intensive explosive eruption.
Cycle Ⅲ was formed between 118 and 111 Ma in the north zone too, mainly distributed in the South Dagang and North Dagang areas. Wells Qg8 and Qg101 are two new drillings and represent this cycle, which were drilled through the Mesozoic strata, unconformable contact with Permian due to the lack of the Jurassic and Triassic strata in the area. Basalt and andesite were recognized within the Cretaceous strata, whereas thick andesitic porphyrite were identified intruded into the Permian strata and dated at about 116 Ma. The igneous rocks developed near the unconformable contact interface of Cretaceous and Permian, which indicated the area may experience intensive denudation before cycle Ⅲ.
Logging response is crucial for lithology identification and lithofacies classification of igneous rocks. The well and seismic data are important for fine stratigraphic division of volcanic reservoir (Chen et al., 2014). For instance, natural gamma ray (GR) and spontaneous potential (SP) curves change obviously at the lithologic boundaries of lava and sedimentary rocks, and the GR, SP and density curves are good indicators for stratigraphic correlation, lithology identification and facies classification of igneous rocks (Wang et al., 2015). Combined with the core samples and logging data, the characteristics of different cycles can be well constrained (Fig. 7).
In Well Qg2 of cycle Ⅰ, the sedimentary rocks are brown siltstone and fuchsia siltstone interlayers in the bottom of the Cretaceous strata, with two thin beds of igneous rocks named basalt developed within brown siltstone layers. The GR curve shows obvious fluctuations where igneous rocks occurred. In the middle part, interbedding of thick brown siltstone, brown sandstone and fuchsia siltstone display smooth features of GR and SP curves. While near the top of Mesozoic strata, the GR curve displays regular and periodic changes caused by the multiple alternations of basalt and mudstone.
Wells X6 and K36 represent the lower and upper parts of cycle Ⅱ, respectively. Gray andesite and basalt developed in the bottom of Well X6, and the relatively straight trend of GR and SP curves indicate the beginning of the cycle. Gray sandstone, siltstone and argillaceous sandstone show overall stable GR and SP trends in the middle part. The GR curve displays a rapid decreasing at the boundaries of the tuffs and sedimentary rocks, and the cycle rhythm occurred in the interior of andesite at the top of the cycle.
Cycle Ⅲ also has distinct lithological association sequences and logging responses. Thick andesitic porphyrite intruded into the Permian strata with a stable trend of SP curve and periodic small amplitude changes of the GR curve. From the bottom up, the typical sequence is andesite with thin sedimentary rocks, then follow by basalt and andesite layers, and thin tuff at last. The GR curve displays a trend from increasing to decreasing, and changing to increase at last, and the SP curve shows a periodic fluctuation at the andesite layers on the near top.
In contrast to the Early Cretaceous, the Late Cretaceous mainly developed intermediate and acid igneous rocks. The principal lithologies include andesite, dacite porphyrite and granite. The andesite and dacite porphyrite were concentrated in the Zaoyuan and Wangguantun areas, and granite was developed in the Yanshan area, with typical wells of Z1532, F22-15, G177, and Y1 (Fig. 8) from the south of the study area.
Both andesite from Well Z1532 and dacite porphyrite from Well F22-15 formed at about 72 Ma. The Wangguantun area is located to the southeast of the Zaoyuan area and separated by the Kongxi fault, where the typical Well G177 developed thick layers of andesite. Since only one set of andesite has been identified in the south zone, we proposed that the andesite of Well G177 belongs to the Late Cretaceous. The Well Y1 within the Yanshan area was drilled into about 900 m thick granite from depth 1 300 to 2 200 m but did not drill through Mesozoic strata after the well completion, the U-Pb dating of granite is 74.0±2.1 Ma. Granite is rarely developed in Late Cretaceous granite in the study area and even in the North China Craton, so it could be an important indicator for our understanding of the magmatic activity.
The current available data show that the wells in the Zaoyuan area share many similarities with those in the Wangguantun area. For instance, they all are lack of the top of Cretaceous strata, and none drillings have drilled into the Early Cretaceous igneous rocks, and also, drilled through the Mesozoic strata. The presence of the Late Cretaceous volcanic rocks indicates that the south zone had suffered less erosion than the north zone, which caused the Early Cretaceous igneous rocks buried even deeper than the current drilling depth in the south zone.
The lithological association sequences and logging responses are simple in the Late Cretaceous magmatism. The GR curves are stable for granite within Well Y1 and andesite within Well Z1532, while the decrease at the top of dacite porphyrite within Well F22-15 may indicate the effect of the lithology boundary with sedimentary rock on it. The GR and SP curves in Well G177 have periodic changes, which are mainly caused by the small differences of magma composition of eruption rhythms.
In the following, Early Cretaceous magmatism and reservoirs developed in Bohai Sea area, Liaoning Province and Huanghua depression are discussed. Late Cretaceous magmatism and reservoirs developed in the Jiaolai Basin, Liaohe depression and Huanghua depression are discussed (Fig. 9).
Previous studies suggested that the subsidence-uplift during Mesozoic were related to the subduction and rollback of the paleo-Pacific Plate (Zhang et al., 2011; Li et al., 2010). Although with a similar tectonic setting, igneous rocks in the Bohai Bay Basin are significantly different from West Liaoning Province. One of the important distinctions is the lack of the Tuchengzi Formation and Zhangjiakou Formation commonly developed in West Liaoning Province, which could be attributed to the uplift of Bohai Bay Basin in the Late Jurassic (Zhu et al., 2019).
The comparison study of the Mesozoic magmatism in Huanghua depression, Bohai Sea with those from West Liao- ning Province shows that the Early Cretaceous magmatism of Huanghua depression may correlate with the Yixian Formation of the other two areas. The lateral comparison chart (Fig. 9) shows that there are three or four cycles of magmatism developed in different areas, which can be subdivided by igneous rock assemblages and sedimentary layers.
The typical geological section of Yixian Formation in West Liaoning Province has been divided into 4 cycles. The stratification boundaries including Zhuanchengzi bed, Dakangbao bed and Jingangshan bed, basic and intermediate rocks were developed in cycle Ⅰ and cycle Ⅱ, whereas intermediate and acid rocks were developed in Cycle Ⅲ and cycle Ⅳ, respectively (Ding et al., 2019; Zhang H et al., 2008).
The nature and formation of the Yixian Formation in the Bohai Sea is poor constrained because it lies on the seafloor. The evidence from the drillings (Cai et al., 2018; Wu et al., 2017; Ye et al., 2017) resulted in a stratigraphic column of the Early Cretaceous igneous rocks in the Bohai Sea area (Fig. 10). Wu et al. (2017) suggested that there were 3 cycles of magmatism developed in the Early Cretaceous, which started from 128 Ma. However, the basalt below the earliest rocks they studied has not been dated yet. According to the regional data, lithologic composition, and deposition sequence, we proposed that there are 4 cycles of magmatism developed in the Bohai Sea area, which is contrasted with that in Liaoning Province. We hold the point that the 3 cycles of magmatism proposed by Wu et al. (2017) identified are the later 3 cycles in the Early Cretaceous, while the basalt below the 128 Ma basalt layer is the first cycle magmatic activity in the area. From cycle Ⅰ to cycle Ⅳ, the lithological sequence in the Bohai Sea display a similar trend as observed in West Liaoning Province, but the volcanoclastic rocks have not been identified in cycle Ⅰ and the magmatism lasted longer in cycle Ⅳ in the Bohai Sea area.
Figure 10. Lateral comparison chart of the typical Early Cretaceous igneous rocks of Huanghua depression and its surrounding areas (145–108 Ma). Intermediate- acid rocks and volcanic sedimentary rocks were the main reservoirs in cycle Ⅱ and Cycle Ⅲ. Data of the Bohai Sea from Wu et al. (2017), data with * from this study. P. Permian; Mz. Mesozoic.
In the Huanghua depression, the general characteristics of lithologic variation trend in the Early Cretaceous are similar to that of the Yixian Formation in the other two areas, but the magmatic sequence is different at some extent. The magmatism in the Huanghua depression could be subdivided into 3 cycles discussed before, therein with similarities between the first two cycles to those from the other two areas, but the main differences are the presence of basalt and the absence of acid rocks in Cycle Ⅲ.
The following discussion of magmatism and igneous reservoirs are based on the magmatic sequences of Huanghua depression, wherein the cycles Ⅲ and Ⅳ of Liaoning Province and Bohai Sea areas are seen as a whole to compare with Cycle Ⅲ of Huanghua depression.
The cycle Ⅰ developed in the Lower Cretaceous and is mainly composed of basalt and andesite. Although the ages of basalts of the cycle Ⅰ in the Bohai Sea area is unavailable, the geological evidence shows that the basalts were formed earlier than 128 Ma. The ages of basalt and andesite are dated to be 136 and 132.3–131.5 Ma in West Liaoning, respectively (Zhang et al., 2016; Cai et al., 2010; Chen and Chen, 1997). Combined with the newly obtained zircon U-Pb age of 140.1 Ma on basalt from Well Qg2 in this study, we proposed that the cycle Ⅰ magmatism activities in these areas share with very similar ages regardless their slightly different lithologies. Basalt is the most widespread lithology of this cycle Ⅰn the three locations we compared, and volcanoclastic rocks and andesite only developed in Liaoning Province. According to the current exploration, no hydrocarbon accumulation has been recognized related to all the igneous rocks in this cycle.
cycle Ⅱ is the main stage of magmatism in Yixian Formation and well-studied. The U-Pb ages of Yixian Formation in West Liaoning Province, Bohai Sea area and in Huanghua depression range from 129.7–122 (Cai et al., 2010; Zhang H et al., 2008, 2005; Wu et al., 2017) to 123.4–118.8 Ma (Zhu et al., 2019; Zhang et al., 2011). The lithological association reflects the alternativity of magmatic eruption, effusion, and sedimentation in this cycle. The magmatism in the Huanghua depression occurred later than the West Liaoning and Bohai Sea areas, and a larger scale of explosive facies rocks developed in the Bohai Sea area and Huanghua depression. The intensive volcanic activities in this cycle resulted to thick layers of vesicular basalt and andesite and formed reservoir space for oil and gas.
Cycle Ⅲ is the last stage of Early Cretaceous magmatism in the North China Craton, with ages varying from 123–111 (Zhu et al., 2019; Guo et al., 2012; Xiao et al., 2008; Zhang H et al., 2008, 2005) to 118–108 Ma (Wu et al., 2017) in Liaoning Province, Bohai Sea and Huanghua depression. The intermediate and acid rocks are the principal igneous rocks developed in this cycle, with similar lithologies in different areas. The except is the lack of acid rocks in the Huanghua depression. It should be pointed out that the thick andesitic porphyrite developed in the South Dagang of Huanghua depression, which is an important sign for Cycle Ⅲ in the depression.
Based on three drillings of Early Cretaceous in the Xiushui Basin located in Liaoning Province, Ding et al. (2019) suggested that the volcanic rocks in cycle Ⅱ and sedimentary rocks in Cycle Ⅲ were the major reservoirs, the mudstone in Cycle Ⅲ has good hydrocarbon bearing conditions. The igneous reservoirs are mainly andesite and volcanoclastic rocks in cycle Ⅱ. This is similar to Liaoning Province. Wu et al. (2017) found that the intermediate-acid rocks developed at the top of Cycle Ⅲ have better oil-gas show than the other sub cycles. They proposed that the lithology is the main factor for different oil-gas bearing grades in different cycles and the porosities of intermediate-acid rocks were better than basic rocks, which may have been caused by the dissolution of anhydrites. In Huanghua depression, we found that the igneous reservoirs are principally intermediate rocks with a few tuffs and volcanic sedimentary rocks in cycle Ⅱ and basic-intermediate rocks in Cycle Ⅲ, while no oil-gas show within the basalt beds in cycle Ⅰ and cycle Ⅱ. Moreover, the andesite reservoirs of the Well Qg8 is a typical rich reservoir type in North Dagang area.
To summarize, intensive magmatism has a closer relationship to the reservoirs, since the cycle Ⅱ and Cycle Ⅲ are the good reservoirs in the Early Cretaceous in the basins of North China Craton. Considering their spatial and temporal distribution, the intermediate-acid rocks in cycle Ⅱ and the basic rocks in Cycle Ⅲ are the main reservoirs in the north zone of the Huanghua depression, and the Cycle Ⅲ is worthy of further study for future exploration.
The Late Cretaceous magma activity in the North China Craton was weak and poorly-constrained, with the ages mainly concentrated in 86 and 70 Ma. The zircon U-Pb ages of Jiaolai Basin in Shandong Province, the Liaohe depression in Liaoning Province, and the Huanghua depression are 86–72 (Zhang J et al., 2008; Meng et al., 2006; Yan et al., 2005), 82–70 (Kuang et al., 2012; Wang et al., 2006; Bing et al., 2003) and 75.7–71.5 Ma (Zhu et al., 2019; Zhang et al., 2011), respectively (Fig. 11). The lithologies of these areas are different from each other. The Jiaolai Basin is mainly composed of basalt and diabase and the Liaohe depression principally developed basalt, dacite, with a small amount of volcanoclastic rocks, while the Huanghua depression developed intermediate-acid rocks and lithology including granite, andesite, volcanoclastic rocks, dacite porphyry, with a small amount of basalt. The igneous rocks in Huanghua depression have more complex lithologies and concentrated zircon U-Pb ages relatively to the other two areas.
Figure 11. Lateral comparison chart of the typical Late Cretaceous igneous rocks of Huanghua depression and its surrounding areas (86–65 Ma). Intermediate and acid rocks were the principal hydrocarbon accumulation reservoirs in Late Cretaceous. *. Unpublished data of Changqian Ma.
According to the early studies and our new data, the reservoirs are mainly related to the intermediate-acid rocks in this stage. There are no oil or gas reservoirs related to the Late Cretaceous basalt in the Jiaolai Basin. Meng et al. (2006) found that the thickness of Late Cretaceous basalt was only about 10 m, and the upper and lower layers were thick sandstone beds. Liu and Wu (2007) suggested that the Cretaceous igneous rocks in the Jiaolai Basin could provide heat source and promote the hydrocarbon generation for the underlying Laiyang Formation and acted as caprocks for the reservoirs. The thick igneous rocks and sandstones stopped the upward migration of oil and gas stored in Laiyang Formation, and the thick overlying layers of sandstone made the Late Cretaceous igneous rocks to be non-reservoir beds.
In Liaohe depression, the dominant igneous hydrocarbon reservoirs were located in the Xinglongtai area and is dominated by andesite and dacite. It has been proposed that the lithology is one of the main controlling factors for reservoirs (Li et al., 2012; Mu et al, 2005). The case studies on the Mesozoic igneous reservoirs in Dawa area in the southeast of Liaohe depression (Li et al., 2020) showed that the effusive basalt have bad storage physical properties. By contrast, basaltic breccia rocks were excellent reservoirs because of their mass of inter- breccia pores and vesicles. In Huanghua depression, several case studies on the igneous rocks of the Zaoyuan and Wangguantun areas (Gao et al., 2015) suggested that the principal reservoirs were the vesicular andesite layers that commonly developed at the top or bottom of the volcanic apparatus (Wang et al., 2004). This study shows that the dacite porphyry and a few volcanic sedimentary rocks are also important reservoirs in the Late Cretaceous, in which the tectonic fractures act as migration pathways and also storage spaces for oil and gas.
Many researchers investigated the main controlling factors of igneous reservoirs, such as the relationship between igneous rocks and source rocks, the influence among tectonic fault zones, lithologies and lithofacies and their reservoir space, the weathering and leaching effects to reservoir spaces (Zheng et al., 2018; Ye et al., 2017). Previous studies on the Bohai Bay Basin and Huanghua depression proposed that the oil and gas were mainly formed in Cenozoic, and the Mesozoic igneous reservoirs have typical characteristics of "young source in the old reservoir" (Zheng and You, 2019; Jin et al., 2012). Jin et al. (2012) summarized three typical hydrocarbon reservoir types to explain how Cenozoic oil and gas charge into Mesozoic igneous rocks in the Bohai Bay Basin, including buried hill type, fault-block type and stratigraphic-lithologic type, in which the first two types are closely related to the faults formed by tectonic activities. In addition, the deep fractures provided channels for the migration of light oil and gas and the small cracks caused by local tectonic movements can also connect the original isolated primary pores and further increased the permeability of rocks (Siler et al., 2018). In short, tectonic activities and faults have been confirmed to have an important influence on hydrocarbon enrichment in "young source in the old reservoir" type reservoirs, which may apply equally to the igneous reservoirs in the Huanghua depression. Hence, we are not going to discuss the significance of fault in isolation in this study, but pay more attention to the relationships of the reservoirs with lithology and reservoir spaces.
Fourteen Early Cretaceous wells and 20 Late Cretaceous wells with oil-gas shows associated with igneous rocks were summarized, and the relationships of reservoir thickness with lithology and oil-bearing grades are shown in Fig. 12.
Figure 12. Oil-bearing grades of Cretaceous igneous rocks of Huanghua depression. (a) Total thickness for Early Cretaceous igneous rocks with oil-gas shows; (b) percentage of oil-gas bearings grades for different lithologies of Early Cretaceous; (c) total thickness for Late Cretaceous igneous rocks with oil-gas shows; (d) percentage of oil-gas bearing grades for different lithologies of Late Cretaceous.
In the Early Cretaceous, basalt and andesite dominate the reservoirs. Basalt reservoirs, with a total thickness of > 230 m, occupy the largest proportion of igneous reservoirs, in which > 50% show fluorescence, > 30% show oil trace and < 10% show oil stain. Andesite layers occupy a thickness of about 50 m and mainly show fluorescence. The share of volcanic sedimentary and tuff are significantly lower than basalt and andesite, though most of the tuff show higher oil-gas bearing grade (Figs. 12a and 12b). The basalt and andesite reservoirs were developed in both cycle Ⅱ and Cycle Ⅲ (Fig. 10), which were the periods of maximum magmatic activities in the Early Cretaceous. Thus, we proposed that the intensive magmatism more conducive to the formation of reservoirs. Besides, the basalt developed in cycle Ⅰ and cycle Ⅱ have no oil-gas show in Huanghua depression (Fig. 10). Therefore, we suggested the basalt in Cycle Ⅲ has large exploration potential in the Early Cretaceous in the Huanghua depression.
In the Late Cretaceous, dacite porphyrite layers are the most developed igneous rock with oil-gas shows, the total thickness is > 700 m with > 70% of them characterized by oil-bearing grades higher than fluorescence. Andesite has a total thickness of > 200 m with most of them show fluorescence. The thickness of volcanoclastic rocks and volcanic sedimentary are < 100 m (Figs. 12c and 12d). Obviously, dacite-porphyrite could be considered as the key exploration objects in the Late Cretaceous due to its thickness and higher grades of oil-gas bearing grades than andesite, and volcanic sedimentary layers are worthy of attention because more than 90% of them show higher grade.
In summary, the intensive Early Cretaceous magmatism may have played a key role in formation of these reservoirs, and basalt in Cycle Ⅲ could be the best exploration target, dacite- porphyrite could be considered as the key exploration object.
Because the oil-gas reservoir spaces directly affect the quality of the petroleum reservoirs of igneous rocks, many researchers have discussed the influence factors to the reservoirs, such as lithologies, lithofacies, tectonic and diagenetic effects and the development of the weathering crust (Wang Y et al., 2018; Zheng et al., 2018). Lithological and lithofacies differences result in different types and intensities of pores. The primary vesicles and fractures laid the foundation for the development and transformation of effective reservoirs. The secondary processes of epidiagenesis are also the key factors, such as weathering and leaching. The formation fluid dissolution can significantly modify the petrophysical characteristics of the reservoir rocks. Meanwhile, tectonism is necessary for the formation of reservoir spaces and the migration, permeation, and accumulation of oil and gas (Zheng et al., 2018; Mao et al., 2015).
For equivalent eruption conditions, the silicic samples show higher tortuosities and produce smaller vesicle sizes and lower permeabilities than mafic samples (Heather et al., 2009). For instance, basalt usually developed larger vesicles than andesites in the core samples in Huanghua depression (Figs. 13a–13c, 13g–13i). If they have similar contact relations with source rocks, primary vesicles, and fractures of basalt could have higher permeabilities and provide better reservoir spaces than andesite, which can explain the basalt has better oil-gas bearing grades than the andesite in the Early Cretaceous.
Figure 13. Different reservoir spaces of igneous rocks. (a), (d), (g), (j) Core images, others are casting thin sections. (a) Well J21-23, 1 589.76 m, large vesicles of basalt; (b) Well J21-23, 1 582.64 m, small amount filled vesicles of basalt; (c) Well J21-23, 1 590.22 m, unfilled vesicles of basalt; (d) Well F22-15, 2 965.26 m, tectonic fractures and corrosion holes of dacite porphyry; (e) Well F22-15, 2 941.37 m, intercrystalline pores of K-feldspar phenocrysts of dacite porphyry; (f) Well F22-15, 2 982.79 m, tectonic cracks of dacite porphyry; (g) Well G177, 2 309.35 m, different types of unconnected vesicles of andesite; (h) Well G177, 2 309.18 m, half-filled vesicle of andesite; (i) Well G177, 2 309.18 m, irregular vesicles of andesite; (j) Well W6, 2 573.25 m, two types of unconnected vesicles of andesite; (k) Well W6, 2 568.73 m, amygdaloidal andesite; (l) Well W6, 2 578.05 m, small vesicles of andesite.
As mentioned above, the basalt reservoirs were concentrated in Cycle Ⅲ, then why the basalts in cycle Ⅰ and cycle Ⅱ were a failure of accumulation? As shown in Figs. 7 and 10, thick sedimentary rocks are laid above the cycle Ⅰ basalt, so if there were no large faults to connect the basalt with Cenozoic source rocks, it's hard for oil and gas to charge into the basalt. As a matter of fact, there was no contact relationship between the cycle Ⅰ igneous rocks and hydrocarbon source rocks have been discovered yet. For cycle Ⅱ, andesite and tuff developed certain reservoirs, similar to cycle Ⅰ, no basalt reservoir was discovered as well. Since only a few core samples have been obtained, we suggest two possible reasons. One possible explanation is that the basalt of this cycle lacks good storage spaces; while the other is the andesite above the basalt has vesicles and dense zones. As a result, the oil and gas would trap in the vesicle zone and formed a reservoir, but the dense zone cut off the migration channel to the basalt. To clarify that, more work related to the 3D seismic interpretation and logging data analysis should be done in the future.
In the Late Cretaceous, the main lithologies of reservoir rocks are dacite porphyry and andesite, and dacite porphyry show better oil-gas bearing characteristics than andesite in the study area (Fig. 12). Casting thin sections show the andesite created different types of primary vesicles, but because few tectonic fractures were developed, most of the vesicles were unconnected. Meanwhile, many vesicles were fully filled with hydrothermal minerals by a secondary process (Figs. 13g–13l), which led to the failure of migration and accumulation of oil and gas. On the contrary, dacite porphyry hardly created vesicles but developed a lot of tectonic fractures, hydrothermal fluid can be transferred through the fractures and dissolved the feldspars, and at last formed inter-crystalline pores and corrosion holes (Figs. 13d–13f). Finally, oil and gas were charged into the secondary pores through the tectonic fractures and formed reservoirs.