Genetic Mechanism Of High Quality Reservoir In Mid-deep Buried Paleogene Clastic Reservoirs In Nanpu Sag

The study of genetic mechanism of high quality reservoir in mid-deep buried clastic reservoirs, regarded as one of hotspots and difficulties of reservoir geology, is of great geological significance in guiding petroleum exploration and exploitation in deep strata. In this paper, a case study on high quality reservoir in mid-deep buried Paleogene clastic reservoirs of Nanpu Sag was carried out. By using a combination of techniques such as core observation, rock slices identification, SEM observation, fluid inclusion analysis, drill core physical property analysis and so on, the distribution characteristics of anomalously high porosity depth intervals(zones) were determined which was based on the idea of ―three lines combining–depth intervals sectioning vertically–zones dividing horizontally‖. Then types of reservoir space as well as their percentages were counted quantitatively, which aimed to determine the genetic types of anomalously high porosity depths intervals(zones). Finally, combined with the diagenetic events and geological evolutional history of diagenetic environment, the genetic mechanisms of different types of anomalously high porosity depths intervals(zones) were discussed systematically and genetic models were established respectively.As a result of the study of anomalously high porosity depths intervals(zones) in Nanpu Sag, anomalously high porosity zones can be divided into primary ones with primary pores absolutely dominating(including pore- increasing ones and primary pore-preserving ones) and secondary ones with secondary pores absolutely dominating(including pore- increasing ones and secondary pore-preserving ones):①pore- increasing primary anomalously high porosity depths intervals(zones) were mainly developed in Dongying Formation, typically in the depth of 2350m-2750 m and 2800m-3100 m in NO.1 and NO.2 tectonic belt of Nanpu Sag; ②pore-preserved primary or secondary anomalously high porosity depths intervals(zones) were mainly developed in the first member of Shahejie Formation, primary one typically in the depth of 3900-4300 m in NO.3 tectonic belt of Nanpu Sag and secondary one typically in the depth of 3200-3600 m in the NO.4 tectonic belt of Nanpu Sag; ③pore-increasing and pore-preserving secondary anomalously high porosity depths intervals(zones) were mainly developed in the third member of Shahejie Formation, pore- increasing dominatingly one typically in the depth of 2730-3300 m in the uplifted side of Liuzan tectonic belt of Nanpu Sag.Sedimentation and diagenesis control collectively the development of anomalously high porosity depths intervals(zones) in Nanpu Sag. The systematic study o f diagenesis suggests that different strata experienced obviously different and complicated diagenetic transformation process which contained multi-period alternation of acidic and alkaline diagenetic environment(see Chapters 4 for detail). Therefore, the analysis of genetic mechanisms of high quality reservoir showed that the effectively coupling of co nstructive dissolution and pore preservation by early hydrocarbon charging and overpressure which can inhibit compaction and cementation control the evolution and development of different types of anomalously high porosity depths intervals(zones): ①the dissolution of feldspar and other aluminosilicate minerals which increases porosity effectively is the major genetic mechanism of pore-increasing primary anomalously high porosity depths intervals(zones) in Dongying Formation; ②high rigid and coarse particle content and early overpressure have an obviously inhibitory effect on compaction, as well early hydrocarbon charging on cementation. These are the major genetic mechanisms of primary pore-preserved primary anomalously high porosity depths intervals(zones) in the first member of Shahejie Formation; ③ early overpressure has an obviously inhibitory effect on compaction and early hydrocarbon charging on cementation which preserve early secondary pore formed by the dissolution of feldspar. These generated secondary pore-preserved secondary anomalously high porosity depths intervals(zones) in the first member of Shahejie Formation; ④pore- increasing dominatingly secondary anomalously high porosity depths intervals(zones) in the third member of Shahejie Formatio n are mainly generated by the dissolution of feldspars and carbonate cements because of the corporate influence of organic acid and meteoric water which can produce plentiful secondary pore.Finally, by means of multi-parameter reservoir classification under the constraints of porosity differences and single well productivity, mid-deep buried Paleogene clastic reservoirs of Nanpu Sag were classified and evaluated and plane distribution of high-quality reservoirs were pointed out. Results indicate that high quality reservoirs ①are widely developed in the first and second member of Dongying Formation in the whole sag; ②mainly distribute in the northern Nanpu Sag in the third member of Dongying Formation; ③are concentrated in the northern Nanpu Sag and the overpressure area in the first member of Shahejie Formation; ④are mainly developed in the uplifted side of Gaoshangpu and Liuzan tectonic belt of Nanpu Sag in the third member of Shahejie Formation.