ABSTRACT

Simple dip areas of continental sedimentary basins feature stratal overlap, gentle changes in slope, uniform dip angles, and minimal fault development. Locating hydrocarbon traps in these environments therefore presents a difficult challenge. This paper analyzes reservoir sand body and hydrocarbon trap development and their distributions within the gently dipping western area of the Songliao basin and discusses the determinants influencing the distribution of reservoir sand bodies and oil and gas bearing zones. Sand bodies within the simple east-dipping western basin margin exhibit thin singular layering, extensive planar distribution, and frequent interbedding with mudstone layers. In the homoclinal slope areas that lack significant fault development, sand bodies served as important hydrocarbon migration pathways. Spatial shifts in the paleosedimentary system caused by basin subsidence and late stage tectonism can help constrain the search for hydrocarbon traps. The spatial patterns of the depositional system influenced the distribution of different types of traps. Southerly prograding delta lobes were subsequently tilted to the east, creating sand bodies that form up-dip, wedge-out lithologic–stratigraphic traps along their western edge. Some of the delta lobes are folded or cut by faults along their peripheral edges, making structural traps and combination traps, the primary types in this region. The delta lobes in the western provenance system, however, dip in a direction parallel to that of sediment supply. Most subaqueous distributary channel sand bodies are continuous into the western source area and lack seal conditions. Lenticular lithologic traps are therefore found only along the outer edges of delta lobes and the adjacent shallow lacustrine environments.

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