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SEQUENCE ANALYSIS OF SOURCE RESERVOIR CAP ROCK ASSEMBLAGES IN SHAHEJIE FORMATION FROM DEEP BED OF WEST DEPRESSION, LIAOHE BASIN, CHINA

ZHENG Rong cai, WU Chao rong (Chengdu University of Technology, China)  
Through the comprehensive analysis of cores, logs and seismic data, six long term stratigraphic base level cycles and six corresponding seismic stratigraphic sequences can be distinguished in the Shahejie Formation , Lower Paleogene from the deep bed of West Depression, Liaohe Basin. In this paper, the relationship between the source reservoir cap assemblages and the long term base level cycle sequences of the Shahejie Formation are discussed in detail, and some knowledge is acquired as follow. That is, (1) in the stratigraphic framework of isochronostratigraphic correlation units of long term base level cycles, the sandbodies developed in the early middle stage of the uprising periods and middle late stage of subsiding periods are the main reservoir bodies, and the huge thick sections of black shale deposited at the turning position from uprising to subsiding of the long term base level are the main hydrocarbon source rocks and favourable regional cap rocks; (2) the characteristics of source reservoir cap rock assemblages developed in each sequence are distinctly different and controlled by the frequency, amplitude of the rising and falling of long term base level cycles, and the rate changes of lake level rising and falling, facies association, depositional sequence or fractionation , accommodation, A/S ratio of each stratigraphic sequence; (3) according to the disposal relationship of source rocks, reservoir rocks and cap rocks in the vertical or horizontal section of the formation, three types of basic assemblage can be divided. What's more, the first assemblage formed during uprising period with high accommodation is usually of the type with source rocks in the up part and reservoirs below, the second one fomed during subsiding period with mid high accommodation usually belongs to the type of source rocks below and reservoirs in the up part, and the third one fomed during the whole process of the base level uping falling cycle with low accommodation and only developed the reservoir cap combination without source rocks. By stacking the second and first basic types or second and third basic types, two complicated assemblage patterns across two stratigraphic sequences are made up and developed along the sequence boundary. It is proposed that the complicated patterns of assemblages are more favourable for reservoir of oil gas than the former three basic types.
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