A Fine Investigation on Geological Conditions for Oil Accumulation in the Paleogene Intersalt Shale Series in the Qianjiang Depression

Abstract

This study dealt with the rhythmic Qian 3410 of the intersalt shale series in the Qianjiang Depression, in which the geological condition for oil accumulation in the rhythmic shale was investigated by using the basic drilling, core and well logging data, as well as data obtained from the thin section identification, SEM, mercury injection, X-ray diffraction and rock freezing heat analyses. The study result indicated that: 1) the intersalt shale series are characterized by TOC in a range of 0.35% - 6.38% (averaging 3.19%), Types I and II1 organic matter, and from immature to mature oil, indicative of a potential for generating a great volume of immature oil; 2) in the target layer, mineral compositions are complicated, which can divide into argillaceous dolomite facies, argillaceous limestone facies, dolomitic mudstone facies, and calcium-mirabilite-filling dolomitic mudstone facies; and 3) of them, the argillaceous dolomite facies is the most contributory, with reservoir spaces consisting of intergranular pores as dominant, as well as rare dissolution pores and seams. It is characterized by high organic abundance (4.23% - 6.38%), high content of brittle mineral (50% - 71%), low content of clay mineral (18% - 33%), high porosity (10.8% - 26.3%) and good oil-bearing properties (S1 distributed in 3.48 - 5.64 mg/g, and S1/TOC value in a range of 348 - 564 oil mg/g). Therefore, the argillaceous dolomite facies was considered the key target series to be explored for the intersalt shale oil in the study area in the coming days.