| Citation: | Guosheng Xu, Deyu Gong, Haifeng Yuan, Changhong Li, Guozhi Wang, Xiaofeng Hu, Juanhua Lin, Jianmin Zhu. Fluid Geochemical Features and Preservation Conditions of Marine Stratum in Typical Structure of Jianghan Plain Area. Journal of Earth Science, 2011, 22(6): 768-779. doi: 10.1007/s12583-011-0226-1
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The evaluation of preservation conditions involves two aspects: objective abilities and practical performance. The objective abilities mainly refer to the basic conditions of oil and gas reservoirs, such as closure property of cap rocks and faults. The practical performance directly reflects the preservation conditions of oil and gas reservoirs, such as alteration and dispersion of surface water (Chen et al., 2003). Many scholars carried out large quantity of research on oil and gas preservation conditions of marine carbonate rocks by some traditional ways, such as distribution and closure property of cap rocks, sealing characteristics of faults, chemical features of formation water, hydrodynamic environment, and sectonic process (Deng et al., 2009; Lou et al., 2008; Tang et al., 2008; Hu et al., 2007; Guo et al., 2003; Cao and Luo, 1996). We have probed a new method to dynamic evaluation of oil and gas preservation conditions of marine strata in multiphase tectonic variation areas from the perspective of paleofluid geochemistry (Wang and Liu, 2009). With this method, this article focuses on the analyses and evaluation of oil and gas preservation conditions of marine stratum in such three typical structures in Jianghan plain area as Jianyangyi, Huchang, and Hongfeng by employing measured geochemical isotopic parameters of fluid.
Located in the eastern part of the Middle Yangtze region, Jianghan plain area is near to the JiangnanXuefeng orogenic zone to the south and the QinlingDabie orogenic zone to the north (Fig. 1). It is a reformed basin experiencing evolution of multiphase basin and tectonic reworking process on the metamorphosed basement of Presinian system during the Jinning movement in the Late Proterozoic (Guo et al., 2006; Xiao et al., 2005). According to the different characteristics of lateral tectonic reworking, nine secondary structures could be divided in the area from the perspective of tectonic origin: Bahong single thrust belt, Zhongxiang imbricated thrust belt, Dangyang decollement folded belt, Huangling uplift, Yichang sustained belt, Jinzhou intrusive zone of bruchfalten, Mianyang intrusive zone of bruchfalten, Daye ramp belt, and Honghu-Tongshan foreland thrust belt. The marine strata were widely distributed in the area. Although much indication of oil and gas has appeared after exploration since 1960s, the substantial breakthrough of oil and gas has not been achieved yet. The reserve of oil and gas has been regarded as a crucial factor of marine oil and gas accumulation in this area by several years' exploration and scientific research.
The structure of Jianyangyi, a dual obduction structure (Figs. 1 and 2), is located near the southwest border of imbricated thrust belt in Zhongxiang. In the depth of 4 116.19 to 4 183.04 m (Maokou Formation) of Well Jianyang 1, the exploration well of Jianyangyi structure, the exploration failed after the completion well test with gas production of 24.84 m3/d and water production of 14.82 m3/d.
The whole lithology of Daye Formation in Lower Triassic in Well Jianyang 1 is dark gray micritic limestone. Gray white calcite vein fills in the host rocks' splits and those calcite consisting of veins are high in idiomorph degree. There co-exists gray white columnar plaster and calcite in some local position. The δ13C value of host rocks in Daye Formation is between -1.084‰ and 3.831‰ (PDB) and that of vein is between -0.245‰ and 2.209‰; the δ18O value of host rocks in Daye Formation is between -17.219‰ and -15.677‰ (PDB) and that of vein is between -16.317‰ and -8.161‰ (Fig. 3). The isotope D-value Δδ13C of the calcite filling in the splits and the host rocks is between -4.076‰ and 0.935‰ and that of Δδ18O is between 0.532‰ and 7.516‰. The δ13C value of vein is either higher or lower than that of the host rocks, while the δ18O value of vein is obviously higher than that of the host rocks. The apparent difference of carbon and oxygen isotope between the vein and the host rocks suggests that the fluid forming veins should not come directly from the adjacent host rocks.
The whole lithology of Maokou Formation in Lower Permian is dark gray micritic limestone. Gray white calcite vein fills in the host rocks' splits. The δ13C value of the host rocks is between 3.449‰ and 4.764‰ and the δ18O value is 3.449‰ and 4.764‰. The δ13C value of the vein is between 2.560‰ and 4.657‰ and the δ18O value is -11.386‰ and -9.274‰ (Fig. 3). The isotope D-value Δδ13C of the calcite filling in the splits and the host rocks is between -0.889‰ and 0.107‰ and that of Δδ18O is between -0.512‰ and 0.176‰. The slight difference of carbon and oxygen isotope of one pairing sample between the vein and the host rocks might indicate that the fluid forming the vein just came from the host rocks, while the clear difference of that of another pairing sample suggests that this part of fluid should be an exogenous one.
A thrust fault exists near the depth of 4 382.00 m of the well and there is a thrust belt between the depths of 4 382.00 and 4 562.00 m. The δ13C value of the adjacent host rocks in the 2nd member of Maokou Formation in Lower Permian existing on the thrust belt is 4.156‰ and the δ18O value is -8.410‰. The δ13C value of the vein is 4.055‰ and the δ18O value is -5.796‰. The isotope D-value Δδ13C of the vein and the host rocks is -0.101‰ and that of Δδ18O is 2.614‰. There exists a slight difference of carbon isotope and obvious difference of oxygen isotope between the vein and the host rocks.
Another thrust fault exists near the depth of 4 782.50 m of the well and there is a thrust belt between the depths of 4 782.50 and 4 910.00 m. The δ13C value of the adjacent host rocks of Wujiaping Formation in Upper Permian existing on the thrust belt is -0.111‰ and the δ18O value is -16.331‰. The δ13C value of the vein is -0.250‰ and the δ18O value is -18.188‰ (Fig. 3). The isotope D-value Δδ13C of the vein and the host rocks is -0.139‰ and that of Δδ18O is -1.857‰. The slight difference of carbon isotope and obvious difference of oxygen isotope between the vein and the host rocks indicates that the fluid forming calcite veins do not come from the host rocks.
Another thrust fault appeara again near the depth of 4 910.00 m of the well. The δ13C value of the adjacent host rocks in the 1st member of Daye Formation in Lower Triassic existing on the footwall of the thrust belt is -1.157‰ and the δ18O value is -17.527‰. The δ13C value of the calcite vein is -0.775‰ and the δ18O value is -18.233‰. The clear difference of carbon and oxygen isotope between the vein and the host rocks indicates that the fluid forming calcite veins should be an exogenous one.
The 87Sr/86Sr values of contemporary normal paleo-seawater of Jialingjiang Formation in Early Triassic, which was simulated by Christoph et al. (2005), are between 0.707 6 and 0.708 2 (Christoph et al., 2005; McArthur, 1994). The 87Sr/86Sr values of contemporary normal paleo-seawater of Daye Formation in Early Triassic, which was simulated by Veizer et al. (1999), are between 0.707 6 and 0.707 8 and that in Early Permian is between 0.707 6 and 0.708 2 (Veizer et al., 1999; Reinhardt et al., 1998). Huang et al. (2008) studied the formation and evolution of strontium isotope of seawater (Late Permian–Early Triassic) in Zhongliang Mountain, Chongqing, and got a result close to Christoph et al.'s, which shows that it also can be applied to South China.
The 87Sr/86Sr values of the host rocks and calcite veins in splits of the 2nd member of Jialingjiang Formation (T1j2, 3 224.60 m) in Lower Triassic are both 0.707 6 (Fig. 3). The 87Sr/86Sr values of the host rocks are close to that of the contemporary seawater, which shows that the host rocks are not remolded by exogenous fluid. The 87Sr/86Sr values of the calcite in splits and that of seawater are also the same, which suggests that the same strontium isotope values of the host rocks and calcite veins are not due to the balance of water-rock interaction but to the fluid filling in the splits coming directly from Jialingjiang Formation itself in Lower Triassic.
The host rocks of the 4th member of Daye Formation (T1d4, 3 429.35 m) in Lower Triassic are gray dolomicrite with two-phase plaster filling in the splits. The 87Sr/86Sr value of host rocks is 0.709 6. The 87Sr/86Sr values of plaster in early and late stages are 0.710 4 and 0.710 3, respectively, which are quite similar to each other. The 87Sr/86Sr values of veins are both higher than that of normal contemporary seawater, which means the fluid forming the veins is an exogenous one rich in strontium.
The 87Sr/86Sr values of host rocks and veins in splits in the 3rd member of Daye Formation (T1d3, 3 595.70 m) in Lower Triassic are 0.708 1 and 0.708 2, respectively. The obvious higher 87Sr/86Sr value of veins than that of contemporary seawater shows that the fluid forming the veins is an exogenous one rich in strontium. The exogenous fluid has experienced fully isotope exchange with host rocks which results in the similar strontium isotope values between the veins and host rocks.
The 87Sr/86Sr values of host rocks in the 1st and 2nd members of Daye Formation are between 0.708 1 and 0.708 2 and the 87Sr/86Sr values of veins in splits are both 0.710 2 (Fig. 3). The obvious higher 87Sr/86Sr value of veins than that of contemporary seawater shows that the fluid forming the veins is an exogenous one rich in strontium.
The 87Sr/86Sr values of host rocks in the 4th member of Maokou Formation (P1m4, 4 176.54 m) in Lower Permian are 0.707 5, that of calcite veins in splits is 0.707 4, and that of calcite veins in splits of host rocks in the 1st member of Maokou Formation (4 315.42 m) in Lower Permian is 0.707 7. The 87Sr/86Sr values of host rocks and veins in splits of Maokou Formation are very close to those of normal contemporary seawater, which means the fluid forming the veins should come from the host rocks itself. This vein is the product of redissolution and reprecipitation on the spot; that is, it should be the product of redissolution and reprecipitation of limestone in Early Permian.
The 87Sr/86Sr values of plaster in splits of host rocks in the 3rd Member of Qixia Formation (P1q3) in Lower Permian, obviously higher than that of contemporary seawater, are 0.709 3, which shows that the fluid forming the veins is an exogenous one rich in strontium.
The 87Sr/86Sr values of host rocks in the 2nd member of Maokou Formation (4 536.70 m) in Lower Permian, which is formed by reappearance of strata due to thrust faults, are 0.707 5. The 87Sr/86Sr values of calcite veins in splits of the host rocks are 0.710 2 (Fig. 3). The similar 87Sr/86Sr values of the host rocks and the normal contemporary seawater suggest the fluid forming veins have some characteristics of rich strontium. The difference in 87Sr/86Sr values between veins and host rocks hints that there lacks isotope exchange between exogenous fluid and host rocks.
The 87Sr/86Sr values of host rocks of Wujiaping Formation (P2w, 4 827.76 m) in Upper Permian, which is formed by reappearance of strata due to another thrust faults, are 0.708 4. The 87Sr/86Sr values of calcite veins in splits of the host rocks are 0.710 2 and that of the normal seawater in Late Permian are between 0.706 7 and 0.708 2 (Shi et al., 2002). The obvious higher 87Sr/86Sr values of veins than that of the normal contemporary seawater imply the fluid forming calcite veins is an exogenous one rich in strontium.
The 87Sr/86Sr values of host rocks in the 1st member of Daye Formation (5 205.82 m) in Lower Triassic, which is formed by reappearance of strata due to another thrust faults, are 0.708 4. The 87Sr/86Sr values of plaster and calcite in splits of the host rocks are 0.710 0 and 0.707 5, respectively (Fig. 3). The obvious higher 87Sr/86Sr values of plaster veins than that of the normal contemporary seawater suggest the fluid forming plaster is an exogenous one rich in strontium. The difference between host rocks and plaster veins indicates there is no experience thorough isotope exchange between exogenous fluid and host rocks. The similar 87Sr/86Sr values of calcite veins filling in splits of host rocks and the normal contemporary seawater as well as the seawater in Late Permian show that the veins forming calcite should come from host rocks themselves or from the underlying strata in Late Permian. However, the D-valued of carbon and oxygen isotope (δ13C is 0.382‰ and δ18O is -0.706‰) show that the fluid forming calcite vein in splits of host rocks should be exogenous. According to the characteristics of strontium isotope, the possible filling process could be inferred, that is, the earlier exogenous fluid rich in strontium filling process. The interaction of fluid rich in strontium and host rocks brought about the increase in strontium isotope values. Later, the fluid coming from stratum itself in Late Permian was refilled.
To sum up, the 87Sr/86Sr values of host rocks and veins of Jialingjiang Formation in Lower Triassic are very close to that of normal contemporary seawater and there is no other intrusive fluid. Except for the 87Sr/86Sr values of veins in the 1st member of Daye Formation in Lower Triassic, the 87Sr/86Sr values of veins of Daye Formation in Lower Triassic to Permian are 0.708 1 to 0.710 4, which are close to that of strata in Early Cambrian.
Huchang structure is located in the northwest of intrusive zone of bruchfalten in Mianyang (Fig. 1). The oil and gas testing result of Well Xia 4 in Huchang structure is water or dry layer in Lower Permian, Carboniferous, and Devonian.
The whole lithology of host rocks in the 2nd Member of Maokao Formation (P1m2) in Lower Permian is dark gray micritic limestone and the splits of host rocks are filled with light gray white fine calcite veins. The δ13C and δ18O values of the host rocks in a pairing sample taken from near 3 004.60 m are 5.125‰ and -5.896‰, respectively; those of calcite veins are 4.364‰ and -12.486‰, respectively (Fig. 4). The carbon and oxygen isotope D-values between veins and host rocks are -0.761‰ and -6.590‰, respectively. The relatively obvious carbon and oxygen isotope difference between veins and host rocks shows that the fluid forming veins do not come from host rocks.
The whole lithology of host rocks in the 1st member of Qixia Formation (P1q1) in Lower Permian is gray micritic limestone and light gray white fine-medium crystal calcite veins fills in splits of limestones. The δ13C and δ18O of the host rocks around 3 259.18 m are -3.178‰ and -7.543‰ and those of calcite veins -0.414‰ and -14.720‰, respectively. The carbon and oxygen isotope D-values between veins and host rocks are 2.764‰ and -7.177‰, respectively. The apparent carbon and oxygen isotope difference between veins and host rocks suggests that the fluid forming veins do not come from the host rocks itself.
The whole lithology of host rocks of Chuanshan Formation (C2c) in Upper Carboniferous is gray micritic limestone and gray white giant crystal calcite veins filled in splits of host rocks. The δ13C and δ18O of the host rocks around 3 300.01 m are -0.605‰ and -8.904‰ and those of calcite veins are -0.987‰ and -10.598‰, respectively. The comparatively apparent carbon and oxygen isotope difference between veins and host rocks might indicate that the fluid forming veins is exogenous.
The 87Sr/86Sr values of host rocks in the 2nd Member of Maokou Formation (P1m2, 3 004.60 m) in Lower Permian are 0.708 6 and that of calcite veins in splits of the host rocks is 0.707 2 (Fig. 4). The 87Sr/86Sr values of host rocks are obviously higher than that of normal contemporary seawater, while the 87Sr/86Sr values of veins are in the scope of strontium isotope of seawater in Late Permian. Both of these meant the fluid forming calcite veins should be from overlying Late Permian. The high value of strontium isotope of host rocks hints that the host rocks has been remolded by exogenous fluid rich in strontium in earlier time before the fluid filling (such as hydrothermal activity). This means that there is a two-period fluid filling in this member, fluid rich in strontium in earlier time and the one in overlying Late Permian later.
The 87Sr/86Sr values of calcite veins in splits of host rocks in the 3rd Member of Qixia Formation (P1q3, 3 105.36 m) in Lower Permian are 0.707 2. The similar strontium isotope of the calcite veins and that of seawater in Late Permian indicate that the fluid came from overlying Late Permian strata; that is, it has the characteristic of upper dissolution and bottom deposition.
The 87Sr/86Sr values of host rocks in the 1st Member of Qixia Formation (P1q1, 3 259.18 m) in Lower Permian are 0.708 7 and that of calcite in splits is 0.709 7 (Fig. 4). The strontium isotope value of veins and host rocks is both higher than that of normal contemporary seawater, while it is similar between the veins and that of fluid in underlying Early Cambrian stratum, which indicates the fluid came from underlying Early Cambrian.
The 87Sr/86Sr values of host rocks of Chuanshan Formation (C2c, 3 300.01 m) in Upper Carboniferous are 0.708 7 and that of calcite veins in splits is 0.707 3. The 87Sr/86Sr values of normal seawater in Carboniferous are between 0.707 5 and 0.708 5 (Reinhardt et al., 1998). The 87Sr/86Sr values of host rocks are a little bit higher than the strontium isotope value of normal contemporary seawater, while the 87Sr/86Sr value of veins is within the scope of strontium isotope value of normal seawater in Late Permian. These suggest the fluid came directly from overlying Late Permian stratum. The high strontium isotope value of host rocks might mean that the host rocks had been remolded by exogenous fluid rich in strontium before fluid filling in Upper Permian.
Hongfeng structure is located in the center of intrusive zone of bruchfalten in Mianyang (Fig. 1) and belongs to fault propagated fold in structural style (Fig. 5). Well Feng 1 of Hongfeng structure has produced water of 0.34 m3/d when oil and gas is tested in Qixia Formation in Lower Permian.
The whole lithology of host rocks of Daye Formation (T1d) in Lower Triassic is dark gray micritic limestone and light gray white fine-medium crystal calcite veins fill in splits of host rocks. The δ13C and δ18O values of the host rocks around 3 137.87 m are 0.164‰ and -6.539‰ and those of calcite veins 0.470‰ and -11.442‰, respectively (Fig. 6). The carbon and oxygen isotope D-values, δ13C and δ18O, between veins and host rocks are 0.306‰ and -4.903‰, respectively. The comparatively apparent carbon and oxygen isotope difference between veins and host rocks shows that the fluid forming calcite veins is an exogenous one.
The whole lithology of host rocks of Badong Formation (T2b) in Middle Triassic is gray-dark gray micritic dolomite and light gray white fine crystal plaster fills in splits of host rocks. The 87Sr/86Sr values of host rocks and veins around 2 700.12 m are 0.709 0 and 0.708 2, respectively (Fig. 6). Study shows the 87Sr/86Sr values of normal seawater of Badong Formation in Middle Triassic are between 0.707 7 and 0.708 0 (McArthur, 1994). The 87Sr/86Sr value of host rocks is clearly higher than that of normal contemporary seawater, while that of veins is normal. These mean that the fluid forming veins should come from Badong Formation in Middle Triassic and not from the adjacent host rocks directly. The high value of strontium isotope ratio of host rocks hints that the host rocks had been remolded by earlier exogenous fluid rich in strontium (hydrothermal activity, for example) before the fluid filled. The close strontium isotope value among host rocks and that of seawater in Early Cambrian and Late Cambrian might result from the full strontium isotope exchange between fluid and host rocks in underlying Cambrian.
The 87Sr/86Sr values of host rocks of Daye Formation (T1d, 3 137.87 m) in Lower Triassic are 0.708 3 and that of calcite in splits of host rocks is 0.709 2. The obvious difference of strontium isotope between veins and host rocks implies that the fluid forming calcite veins is an exogenous one, which influences host rocks and makes its strontium isotope ratio of host rocks relatively rise. The strontium isotope of host rocks and veins is not balanced and the strontium isotope ratio of veins shows that the fluid should come from underlying Cambrian.
The 87Sr/86Sr of host rocks and veins of Huanglong Formation (C2h, 4 083.75 m) in Upper Carboniferous are 0.709 0 and 0.709 2, respectively, while that of normal contemporary seawater is between 0.707 5 and 0.708 5 (Shi et al., 2002; Huang, 1997). The strontium isotope ratio of veins and host rocks is both higher than that of normal contemporary seawater but similar in underlying Cambrian, demonstrating that the fluid was from underlying Cambrian.
It was believed that there existed two fluid systems in Well Jianyang 1 after multidisciplinary analysis of carbon, oxygen, and strontium isotope of veins and host rocks in different depths here (Fig. 2). The upper fluid system developed from Jialingjiang Formation and the one from Daye Formation-Permian. There were two periods of fluid filling in lower system. In the earlier fluid filling, the fluid in Lower Cambrian and overlying stratum of Early Triassic was interconnected. The fluid flowed from bottom to top along the fault and the oil and gas preservation conditions of Palaeozoic stratum became worse on the whole. The fluid in late period might be mainly from Early Triassic or Late Permian stratum. The lack of traces of the fluid from underlying Later Palaeozoic suggested that Jianyangyi structure possessed relatively good fluid preservation conditions on the whole during the late-period filling.
It was judged that there existed at least two periods of fluid filling through the comprehensive analysis of carbon, oxygen, and strontium isotope of veins and host rocks in different layers of Well Xia 4 (Fig. 7).
It was a kind of filling of fluid rich in strontium in the early stage. This fluid, represented by strontium isotope ratio of veins in sample P1q1, came from Early Cambrian stratum. This meant that the Early and Late Palaeozoic strata were interconnected and oil and gas preservation conditions in underlying Lower Palaeozoic were damaged when the fluid filled in Permian reservoir in the early period. The fluid filling in Permian reservoir in the late period only came from the contemporary strata and lacked some Palaeozoic fluidogenous features of anatectic origin, which hint that the fluid of Early and Late Palaeozoic strata in Huchang structure was not interconnected and the fluid preservation conditions of Upper and Lower Palaeozoic was good. Therefore, the strata of Upper Palaeozoic possessed good preservation conditions; however, there was no possibility of oil and gas existence in Lower Palaeozoic due to the damage in the earlier period.
The study on carbon, oxygen, and strontium isotope features of host rocks and veins in different layers shows that the host rocks around 2 700.12 to 4 083.75 m of Well Feng 1 were all remolded by fluid rich in strontium from underlying Cambrian. The fluid in strata of Early Triassic and underlying Cambrian of Hongfeng structure was interconnected and Palaeozoic possessed relatively bad preservation conditions in the early period. The 2nd fluid filling existed in some local places in the late period and its fluid came from Middle Triassic stratum itself (Fig. 5). There might have better fluid preservation conditions to Palaeozoic in the late period. However, due to the damage to oil and gas in the early period, although the fluid preservation conditions today have been improved, there was still no possibility of oil and gas existence. The strontium isotope features of veins filling in splits in Late Permian and Triassic were similar to that of marine carbonatite in Middle Triassic or Cambrian and there lacked abnormal strontium isotope resulting from the mix of ambient freshwater rich in strontium. These suggested that the Upper Palaeozoic and the overlying strata should possess relatively better fluid preservation conditions.
After studying the fluid geochemical features of marine strata in the three typical structures mentioned above, the writer considers that the fluid of Lower Palaeozoic and the overlying Middle and Lower Triassic were interconnected in the early period. Oil and gas effused to overlying strata along faults or splits, and the paleo-oil and gas pools were damaged. Lower Palaeozoic and the overlying strata were not interconnected in the late period. Good as the fluid preservation conditions were, productive oil and gas pools could not be formed because hydrocarbon-generation fastigium of source rocks in Lower Palaeozoic had passed. The exploration drilling of Well Paishen 1, a new exploration well in Jianghan Plain area, proved the above opinion. Well Paishen 1 was located on the Paizhou structure of intrusive zone of bruchfalten in Mianyang (Fig. 1). The layer of bottomhole was Dengying Formation in Sinian and hydrocarbons were not tested out in Lower Palaeozoic.
Two reasons could explain why there were no oil and gas pools in Permian and Triassic in the research area. Firstly, when connecting with overlying Palaeozoic strata, the fluid also linked with earth surface in the early period. Oil and gas were not fully filled and preserved in traps. While when the preservation conditions of fluid were good in the late period, the hydrocarbon-generation fastigium of source rocks of Permian and Triassic had passed. Secondly, although paleo-oil and gas pools had been formed when the fluid connected with that of underlying Palaeozoic in the early period, they were damaged by multiphase tectonic activities later. When the preservation conditions became better later, the hydrocarbon-generation fastigium of source rocks had passed and there was no hydrocarbon generation anymore.
(1) There existed two-period fluid fillings in the reservoirs of Middle and Lower Triassic to Permian in Jianghan Plain area. The fluid filled in the early period came from underlying Cambrian. The fluid in Middle and Lower Triassic interconnected with that in underlying Cambrian at this moment and the preservation conditions in Lower Palaeozoic were bad. While lacking some Palaeozoic fluidogenous features of anatectic origin, the fluid filled in the late period only came from the strata of contemporary or adjacent age. Now the fluid in Lower Palaeozoic did not interconnect with that in Middle and Lower Triassic to Permian with good preservation conditions of the whole marine strata. However, because the hydrocarbongeneration fastigium of marine source rocks had passed or the paleo-oil and gas pools in the early period had been damaged, there was no hydrocarbon product from the marine strata of Palaeozoic to Triassic in the research area.
(2) The failure of exploration drilling of marine strata in Well Paishen 1 proved the writers' point as well as the reliability of the approach—"the geochemical features of fluid could judge the preservation conditions of fluid indirectly". However, this approach was also limited in that although one of dynamic evaluation methodologies to preservation conditions of fluid, it could only have a very brief recognition of time. For example, only the early and late fluid fillings but not the exact geologic periods could be recognized in this paper. Therefore, the exact inflection time of dynamic change of fluid preservation conditions could not be defined either.
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Guosheng Xu, Deyu Gong, Haifeng Yuan, Changhong Li, Guozhi Wang, Xiaofeng Hu, Juanhua Lin, Jianmin Zhu. Fluid Geochemical Features and Preservation Conditions of Marine Stratum in Typical Structure of Jianghan Plain Area. Journal of Earth Science, 2011, 22(6): 768-779. doi: 10.1007/s12583-011-0226-1
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