Theoretical progress and practice of refined oil and gas exploration in mature exploration area of Jiyang Depression

Jiyang Depression， as one of the mature exploration areas in Bohai Bay Basin， has been explored and developed for over 60 years. It is urgent to solve the theoretical understanding problems such as oil and gas resource potential， exploration direction， and accumulation law to realize the sustainable development of oil and gas exploration. The evolution process of hydrocarbon generation and hydrocarbon generation potential of different types of source rocks were compared based on the analysis and testing of the sedimentary environment and organic geochemical characteristics of a large number of source rock samples in the primary hydrocarbon-generating strata and the hydrocarbon generation simulation experiment. It is found that the source rocks in the saline environment have the characteristics of high ancient productivity and low activation energy， and a “saline and hydrocarbon-rich” of hydrocarbon generation and expulsion mode was established in the continental lake basin. It breaks through the traditional understanding that source rock in freshwater environments is the primary source of oil and gas. It is pointed out that “strong cementation in early stage and strong dissolution in late stage” are the essential reasons for the formation of a large number of secondary dissolution pores in the deep reservoirs by analyzing the relationship between porosity and acid-base properties of formation fluid during diagenetic evolution of deep reservoirs. The genetic problem of effective reservoirs of deep clastic rocks under a high geothermal background is solved. The paper summarizes and puts forward the orderly distribution of lithology， structure-lithology， lithology-structure， and tectonic reservoirs from subsag to slope zones based on the exploration practice in Jiyang Depression. Therefore， the theoretical understanding of refined oil and gas exploration in mature exploration areas of “saline and hydrocarbon-rich， acid-base control， and orderly accumulation” has been innovated， and the potential of remaining resources has been defined. The direction of deep exploration has been pointed out， so oil and gas exploration has changed from passive transfer to active exploration. The division and optimization method of the “exploration layer unit” suitable for the refined exploration stage is innovatively established， and a “seven-step” refined exploration and evaluation process is formed for mature exploration areas. The paper points out the critical exploration directions and fields with increasing reserves in the future， which provide essential guarantees for the long-term benefit and sustainable development of the oilfield based on the new geological understanding of Jiyang Depression and the results of the latest round of oil and gas resource evaluation.