Syn-rift sedimentary evolution and hydrocarbon reservoir models in a graben rift sag, Songliao Basin, Northeast China

Abstract

Identifying hydrocarbon reservoirs in lacustrine rift basins is a challenging task. This study uses seismic, well, and core data to analyse the sequence stratigraphy and describe the depositional systems and hydrocarbon reservoir models in the syn–rifted successions of Sujiatun sag in order to locate prospects in the Late Jurassic–Early Cretaceous continental rift basin. SQ2 is composed mainly of fan–delta and shallow lacustrine depositional systems. SQ3 and SQ4 consist of shallow lacustrine and semi–deep lacustrine depositional systems. SQ5 and SQ6 are composed of braid-delta and shallow lacustrine depositional systems, forming favourable reservoir sand bodies. Data obtained from thin section observation and physical property analysis show that the physical properties of braid-delta reservoirs are better than those of fan-delta reservoirs in terms of hydrocarbon accumulation. Geochemical analysis of the mudstones in SQ3 and SQ4 shows that they are excellent source rocks. Analysis of the conditions and characteristics under which the hydrocarbon reservoirs formed, indicate that Sujiatun sag models can be divided into six categories: a tilted fault–block hydrocarbon reservoir model, Syn trending fault–block hydrocarbon reservoir model, reverse trending fault hydrocarbon reservoir model, fault-lithologic hydrocarbon reservoir model, an updip pinch-out hydrocarbon reservoir model, and a lenticular lithologic hydrocarbon reservoir model. Research indicates that the distinctive depositional systems reflect three episodic syn–rifted stages. In particular, the rift climax influences the development of source rocks. Well-sorted sedimentary conditions and a large number of dissolution pores are key to improving a reservoir. The types and distribution of hydrocarbon reservoirs vary greatly over different structural stages and structural zones. Fault activity and a combination of different faults types, reservoirs, and source rocks jointly determine the enrichment degree of the hydrocarbon accumulation in different reservoirs. This study analyses the sedimentary evolution and hydrocarbon reservoirs within the syn–rifted successi to provide useful insight for oil and gas exploration in syn–rifted basins and may also be effective for use in areas in which wells or outcrop data are lacking.