Abnormal pressure genesis and its relationship with continental shale oil accumulation in Paleogene, Jiyang Depression, Bohai Bay Basin

Abstract

In order to explore the effect of abnormal pressure on shale oil accumulation in the Jiyang Depression, Bohai Bay Basin, the formation pressure distribution, muddy shale lithofacies, hydrocarbon generation evolution of organic matters, clay mineral conversion, hydrocarbon retention, and the distribution of secondary pores and microfractures in each of four exploration wells specially for Paleogene shale oil were studied by total rock X-ray diffraction analysis, conventional pyrolysis analysis on hydrocarbon generation potential, systematic analysis on organic geochemistry, and other methods. In the study area, the lithofacies association in middle and lower submembers of the third member (Es3middle and Eslower)3, and upper submember of the fourth member (Es4upper) of Paleogene Shahejie Formation adjacent to subsag center is mainly carbonate rocks, while near sag margin, there are primarily mixed sediments of clastic rocks/chemical rocks. Based on the data from sonic, density and drilling loggings, pressure distribution and abnormal pressure characteristics in single well could be identified effectively; in addition, the factors that affect formation pore pressure in organic shale containing major association of carbonate and clay minerals could be uncovered. Comprehensive application of geochemical evidences confirmed that Paleogene overpressure in the Jiyang Depression was mainly originated from hydrocarbon generation pressurization in mature muddy shale. Self-source overpressure intervals in muddy shale are in correspondence with the zones of secondary pore development and peak hydrocarbon retention.

Keywords

• continental shale oil
• fine-granular sedimentary rocks
• abnormal pressure
• sonic logging
• bohai bay basin