Abstract
The Bohai Bay region is a primary accumulation area of oil and gas in offshore China, in which overpressure commonly occurs in the Paleogene strata; the analysis on distribution characteristics and genetic mechanisms of the overpressure would provide geologic evidences for making plans of well drilling and logging as well as oil and gas exploitation; additionally, it could lay the geological foundation for studying how overpressure controlled hydrocarbon accumulation. Based on research, the overpressure of the study area starts from the second member of the Dongying Formation and ends in the third member of the Shahejie Formation. The distribution of overpressure is mainly controlled by the sag–salient tectonic framework within the basin, which means overpressure mainly develops in sags or slopes; however, high areas stay normal pressured. In the study area, pressure develops around Bozhong Sag and in northern Liaodong Bay reaches the peak. The genetic mechanisms of overpressures in the Paleogene reservoirs are mainly disequilibrium compaction, hydrocarbon generation of the organic matter, fluid charging, and transmission or the superimposition of the former two. Different strata have different genetic mechanisms of overpressure. The chief genetic mechanisms for the generation of overpressure of the Dongying Formation are disequilibrium compaction while the genesis of the formation of overpressure in the Shahejie Formation is more complicated in some extent. The first member of the Shahejie Formation dominated by disequilibrium compaction and hydrocarbon generation of the organic matter plays a supplemental role, while the second member of the Shahejie Formation, as the primary reservoir strata, is dominated by fluid charging and transmission, and the third member of the Shahejie Formation is the main source rock interval; its overpressure is closely related to hydrocarbon generation. Each contribution ratio for overpressure forming by different genetic mechanisms has been judged and figured out quantitatively according to geological, geophysical, and geochemical characteristics of the target strata.
Highlights
The phenomenon of overpressure commonly exists in the main hydrocarbon basins worldwide (Xu et al, 2004)
In terms of oil and gas generation, overpressure can decrease the maturity of source rocks and delay their thermal evolution process (Hao, 2005)
The Bohai Bay region is a primary accumulation area of oil and gas in offshore China, overpressure commonly occurs in Paleogene strata
Summary
The phenomenon of overpressure commonly exists in the main hydrocarbon basins worldwide (Xu et al, 2004). The Bohai Bay region is a primary accumulation area of oil and gas in offshore China, overpressure commonly occurs in Paleogene strata. The studies on genetic mechanisms and distribution characteristics would provide geologic evidences for making plans of well drilling and logging as well as oil and gas exploitation, and it could improve the theory of overpressure controls hydrocarbon accumulation. Based on rock types, logging data, interpretation results, and oil test results of abnormal pressure interval, combining such data as source rock indices of thermal evolution, crossplot of velocity, density, and vertical effective stress, the three mechanisms of disequilibrium compaction, hydrocarbon generation of the organic matter, and fluid charging and transmission of the working area are comprehensively analyzed, from the single well to the whole block of the Paleogene reservoirs, from mudstone layers to permeable reservoirs. The third member of the Shahejie Formation is the main hydrocarbon source intervals of the study area (Guo, 2016) and some sections with deeper burial depth in the third member of the Dongying Formation can become favorable hydrocarbon source intervals (Figure 2)
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