Evolution characteristics of the upper Paleozoic source kitchen and its controlling effects on hydrocarbon accumulation in the Paleozoic petroleum system in Huanghua Depression, Bohai Bay Basin, China

Abstract

Significant breakthroughs in oil and gas exploration have been made recently in Dagang Oilfield of the upper Paleozoic petroleum system in Huanghua Depression of the Bohai Bay Basin, which is affected by multi-stage tectonic activities. However, the processes of hydrocarbon expulsion and accumulation are complicated and still unclear. In order to solve these problems, core observation, cutting log, logging analysis and geochemical testing of source rocks are used to study the evolution of the source kitchen and its controlling effects on hydrocarbon accumulation. The results show that: The threshold of coal, carbonaceous mudstone and mudstone is 2000, 2100 and 2150 m, respectively, with corresponding vitrinite reflection (Ro) value of 0.68%, 0.7% and 0.72%, respectively. When the threshold is reached, the source rock will experience a rapid process of hydrocarbon expulsion, after which the rate of hydrocarbon expulsion will continue to decline. The hydrocarbon expulsion efficiency of the mudstone is lower than that of carbonaceous mudstone and mudstone；The maximum hydrocarbon expulsion intensity in southern Huanghua (21 × 108 and 22.77 × 108 m3) was much higher than that in northern Huanghua (2.96 × 108 m3) at the end of Jurassic and Cretaceous. At the end of the Paleogene, the difference of hydrocarbon expulsion intensity between the northern area(58.09 × 108 m3) and southern area (78.26 × 108 m3) decreased; The Mesozoic and Cenozoic tectonic events and the migration of the subsidence center in the Huanghua Depression controlled the hydrocarbon expulsion process. Moreover, two main control effects of the source kitchen on oil and gas reservoirs are identified. First, the source kitchen controls the distribution of oil and gas reservoirs, with the size and frequency of these reservoirs decreasing with distance from the source kitchen. Second, the source kitchen controls the hydrocarbon accumulation process. The two large-scale hydrocarbon expulsion periods of the source kitchen are both important periods of oil and gas accumulation. Oil and large-scale gas were mainly charged during the early and late period, respectively. In addition, we summarize three types of source–reservoir matching relationships and hydrocarbon supply models of the source kitchen in the Paleozoic petroleum system and discuss the exploration significance of the lithologic reservoirs within the source.