Oil And Gas Accumulation In Liaozhong Sag

Bohai bay basin is one of the most important petroleum and oil producing areas in China, whose annual production has reached30billion tons in2011. Great success has been made in the exploration of the Liaodong Bay during the past30years, which proved that it is one of the hydrocarbon-rich sags in the Bohai Bay basin. After several decades of exploration, a host of oil and gas fields have been discovered one after another near both sides of the Tanlu strike-slip fault zone and its derivative fault zones in Liaodong Bay such as SZ36-1oil field, JX1-1oil and gas field, JZ25-1oil and gas field, etc. Being one of the most significant oil and gas producing areas, Liaodong Bay still has great exploration potential.Thanks to the efforts made by the several generations of Chinese geologists, the petroleum geology conditions in the Liaozhong sag and its neighboring area have been studied comprehensively. However, with the increasing abundance of the research material and the ever-promoting exploration degree, there still exists some aspects which needs further study:1) With the ever-promoting exploration degree, a large number of new oil and gas fields have been discovered in the Liaodong Bay during the last few years. It is necessary to fully use the geological information of these new oil and gas fields to strengthen the source rocks evaluation and the oil and source rock correlation in the Liaozhong sag.2) It is necessary to study the reservoir characteristics in Liaozong sag and its neighboring area in an overall viewpoint (both macroscopic view and microscopic view) so as to grasp the regularity of the reservoir development in vertical direction and lateral direction. The influence on the oil and gas accumulation imposed by the source-reservoir-cap assemblage should also be analyzed.3)There are still some study areas needs to be improved in the Liaodong Bay Area, such as the distribution characters of the passage system, their different contribution to the hydrocarbon migration by the different passage systems, the micro-architecture of the same type of passage system which locates in different structural situations.4) The quantitative analysis of the oil&gas accumulation process should be incensed, mainly focusing on the hydrocarbon injection period and hydrocarbon accumulation period. Finally, it is necessary to summarize the reservoir formation patterns and their main controlling factors.According to the four problems above, the TOC, types, maturity and thermal evolution history of the four sets of source rocks have been systematically evaluated. It is found that the source rocks in the E3d3are the major source rocks to generate the oil with a low maturity, the source rocks in the E2S1are the secondary source rocks to generate the oil with a normal maturity, and the source rocks in the E23are the major source rocks to generate the oil with a normal maturity. The source rocks in the south and middle sub-sags have a better hydrocarbon generation ability than the north sub-sag does.The geochemistry characteristics of the sources rocks in the typical oil&gas fields and the oil-gas and source correlation has proved that the oil&gas fields locating in the Liaoxi Salient have accepted the contribution of the source rocks from both the Liaoxi Sag and the Liaozhong Sag. For the oil&gas fields locating in the Liaozhong Sag, the oil source mainly comes from the source rocks in their neighboring area.The sedimentary facies controlled the development of the reservoir in Liaozhong Sag. The dominant sedimentary facies of the reservoir in Dongying formation is delta front sub-facies while the dominant sedimentary facies of the reservoir in Dongying formation is fan delta front sub-facies. From the Minghuazheng formation to Shahejie formation, the reservoir physical property becomes worse. In the north and middle parts of the study area, the abnormal high pressure developed widely. The combination of the shale strata and its abnormal high pressure has greatly strengthened the sealing ability for the hydrocarbon accumulation.The research on the passage systems proved that the faults have played the most important role for the hydrocarbon migration. The fault activity rate at its active stage and sealing property at its resting stage have resulted in the differential accumulation of the oil and gas in the traps which are characterized by clear distinction between the primary and the secondary as well as integration of motion and quietness. It has played a significant role in controlling oil and gas migration and accumulation.The hydrocarbon accumulation process of five oil and gas fields(JZ20-2, JZ20-2N, JZ31-6, JZ21-1and LD27-2oil&gas fields) have been analyzed. The modes of hydrocarbon reservoir formation in Liaoxi salient could be generalized as follows-the hydrocarbon comes from the source rocks in both Liaoxi Sag and Liaozhong Sag, the shale strata and the abnormal overpressure controls the sealing of the traps. The hydrocarbon accumulated in several different formations vertically. The modes of hydrocarbon reservoir formation in Liaozhong sag could be generalized as follows-hydrocarbon source acts as "soil", oil and gas as "nutrient", hydrocarbon expulsion relies on "roots", hydrocarbon migration relies on "trunks", reservoir forms in "brunches" and the whole process follows the pattern that "hydrocarbon accumulates in strike slips and oil and gas reservoir forms like the growth of trees".In sum, the petroleum geological characteristics and the hydrocarbon accumulation regularity in the Liaozhong sag have been concluded including the characters and distribution of the high quality source rocks; the patterns of the source-reservoir-cap assemblages and their relationship with the hydrocarbon migration and accumulation; the analyzing of the hydrocarbon accumulation and its controlling factors, which has a great significance to exploit the potentialities of the Liaodong Bay Area and also has a reference function for the other offshore oil and gas bearing areas in China.