Genetic Mechanism And Distribution Patterns Of Medium-deep High-quality Clastic Reservoirs Of Paleogene In Jizhong Depression

The study of origin of high-quality reservoir in mid-deep clastic reservoirs of fault basin has always been the most popular research content of petroleum exploration, and the geological backgrounds of different tectonic belts, the means and intensity of physical and chemical process are different, So the genesis and distribution of high-quality reservoir are also different. In this paper, a case study on mid-deep buried clastic reservoir of the 4th and 3rd members of Shahejie Formation of Paleogene in Jizhong Depression was carried out. With a combination of drilling data, logging data, grain size analysis, thin section analysis, casting thin sections analysis, physical property analysis, scanning electron microscope analysis and fluid inclusion analysis, the characteristics of sedimentation, diagenesis, formation pressure, reservoir physical property and its controlling factors of high-quality reservoir were conducted systematically, the genetic mechanisms of high-quality reservoir in different tectonic belts were analyzed and the genetic modes of high-quality reservoir of rifted lacustrine basin were established, Finally, the prediction of horizontal distribution and physical property distribution of high-quality reservoir was conducted.There are mainly four kinds of sedimentary facies including fan delta, braided stream delta, sublacustrine fan, nearshore subaqueous fan developed in mid-deep high-quality reservoirs of Paleogene in Jizhong Depression. The sedimentary facies of different tectonic belts are different. Braided stream deltas are developed in gentle-slope zone, and nearshore subaqueous fans and fan deltas exist in steep-slope zone, while sublacustrine fans, fan deltas and braided stream deltas are in sub-sag zone. The formation pressure of mid-deep reservoir of Paleogene in Jizhong Depression is characterized by a large-scale weak–middle over pressure and partial strong overpressure; The formation pressure evolves successively within different geological time, and there exists a difference in formation pressure among different tectonic belts within the same period. The sub-sag zone has the strongest formation pressure, then the steep-slope zone follows, and the gentle-slope zone has the weakest formation pressure in Baxian and Raoyang Sag; While the formation pressure in steep-slope zone and gentle-slope zone is stronger than that of sub-sag zone in Langgu Sag.Mid-deep reservoir of Paleogene in Jizhong Depression mainly belongs to the middle diagenetic stage of A, but the types of diagenesis, diagenetic evolution sequence and diagenetic facies sequence of different tectonic belts are different. Diagenesis types of gentle-slope zone mainly consist of compaction and cementation and weak dissolution. Diagenesis types of steep-slope zone of Langgu Sag are mainly composed of compaction and matrix recrystallization, and the diagenetic evolution sequence is compaction/ early calcite cementation-feldspar dissolution/ quartz overgrowth/authigenic kaolinite precipitation-hydrocarbon injection-carbonate cementation/anhydrite cementation/quartz dissolution-pyrite cementation. There exists a difference in the diagenetic environments of Sag zones due to tectonic evolution of the basin. The sub-sag zone of Langgu Sag experienced the diagenetic environment with ―early weak alkaline-early acid-middle alkaline-late acid-late weak alkaline because of late uplift; The sub-sag zone of Raoyang Sag experienced continuous burying and its diagenetic environment evolution is ―early alkaline-early acid-late alkaline-late acid‖, thus forming different diagenetic evolution sequence. The diagenetic evolution sequence of Liuquan-Caojiawu tectonic belt is ―compaction/pyrite cementation-feldspar dissolution/quartz overgrowth/authigenic kaolinite precipitation-early carbonate cementation/quartz dissolution-feldspar/ weak dissolution of carbonate cement-late carbonate cementation/pyrite cementation‖; While the diagenetic evolution sequence of the sub-sag zone in Raoyang Sag is compaction/early calcite cementation-feldspar dissolution/quartz overgrowth/authigenic kaolinite cementation-hydrocarbon injection-late carbonate cementation/quartz dissolution-late pyrite cementation.By introducing the ―the normal evolution trend lines of maximum porosity and average porosity‖, combined with the characteristics of porosity evolution envelop curve, the reservoir porosity-depth profile of different tectonic belts are divided into different porosity zones(intervals). By defining the reservoir with porosity higher than maximum porosity of normal evolution trend line as the high-quality reservoir, and based on the division of 50% of relative content of secondary pore as the standard of reservoir types, the study shows that high-quality reservoir with primary pore predominating exists in the gentle-slope zone, and its relative content of secondary pore is less than 25%; The reservoir within the shallow depth of steep-slope zone belongs to high-quality reservoir with primary pore predominating, but its relative content of secondary pore can reach 37.5%. And the deep-buried reservoir of steep-slope zone is high-quality reservoir with secondary pore predominating, of which the relative content is 65%; The characteristics of Maxi sub-sag zone, Suning-Dawangzhuang tectonic belt, Liuchu- Yangwuzhai tectonic belt is similar to those of steep-slope zone with high quality reservoir with primary pore dominant in shallow depth and high quality reservoir with secondary pore predominanting within deep depth, of which the relative contents of secondary pores are 32-49% and more than 60% respectively.By comparing the cementation content, grain size and sorting characteristics, matrix content, sedimentary facies types, formation pressure characteristics and reservoir physical properties, It is concluded that sedimentation, diagenesis, formation pressure and hydrocarbon injection are major factors controlling physical property of high-quality reservoir. The further conclusion is that the matching relations between the above different geological factors of different tectonic belts contribute to the development of high-quality reservoirs with their own characteristics. The high-quality reservoir of gentle-slope zone of Jizhong Depression belongs to primary high-quality reservoir with the genetic mechanism of primary pore preservation by hydrocarbon injection. The high-quality reservoir of steep-slope zone in Langgu Sag is the successive dissolution pore –fractured reservoir with the combined genetic mechanism of porosity preservation by favorable constituents and porosity –increasing by tectonic fractures; The high-quality reservoir in shallow depth of steep-slope zone of Baxian Sag is primary high-quality reservoir with the genetic mechanism of porosity-increasing by Organic acid dissolution, and that in deep depth is secondary high-quality reservoir with the combined genetic mechanism of porosity-increasing by Organic acid dissolution and porosity preservation by overpressure and hydrocarbon injection. The high-quality reservoir of sub-sag zone in Langgu Sag belongs to secondary high-quality reservoir with the genetic mechanism of porosity-increasing by meteoric water and organic acid dissolution; The high-quality reservoir in shallow depth of sub-sag zone in Raoyang Sag is primary high-quality reservoir with genetic mechanism of porosity-increasing by organic acid dissolution, and that in deep depth is secondary high-quality reservoir with the combined genetic mechanism of porosity –increasing by organic acid dissolution and porosity-preservation by overpressure and hydrocarbon injection. On the basis of the analysis of the genetic mechanisms of high-quality reservoirs within different tectonic belts, the genetic modes of high-quality reservoirs of the basins experiencing late uplift and continuous burying respectively were established.Based on the single factor analysis and major controlling factor analysis of controlling factors of reservoir physical property, combined with the evaluation methods dominated by three parameters under the constraint of productivity, reservoir classification and evaluation was carried out and distributions of high-quality reservoir was predicted depending on the superimposing graphs. The results show that high-quality reservoirs is mainly located in the areas with favorable sedimentation, favorable diagenetic facies sequence and overpressure; Under the guidance of genetic modes, the physical property distribution of high-quality reservoir with different tectonic zone was predicted by the calculation of pore-increasing content and pore-preservation content of different genetic mechanisms quantitatively.