The Songliao Basin is geologically unique with respect to the large volumes of organically rich nonmarine source rock, high geothermal gradients, structural focusing of major depositional system, and tectonic inversion coincident with the main locus of clastic deposition. Much of the basin has reached a mature stage of exploration, with cumulative production of over 10 billion barrels, but new exploration plays are being developed in several parts of the basin, including deeper targets in the north and a major emphasis on gas in the southern sector.

The tectonic evolution of Songliao Basin was typical of fault-bounded continental rifts. Complex Hercynian tectonic grain influenced the horst and graben topography of the synrift Upper Jurassic and Neocomian basins. As many as 40 discrete grabens and half grabens each show their own distinct patterns of deposition, with varying exploration potential. Alluvial-fan and fan-delta conglomerates, volcaniclastics, and coarse sandstones flank the faulted subbasin margins, with finer-grained coal-bearing strata of deltaic and lake-margin environments along the gently sloping ramped margins of half grabens. The axial depressions were filled with organic-rich lacustrine sediments. Deposition in the Jurassic-Neocomian subbasins was characterized by narrow facies zones and abrupt lateral facies changes. There is a suggestion that half-grabens were dominated by fluvial systems whereas full grabens developed lacustrine or coaly depocenters.

As individual rift basins filled, the area of deposition expanded, and by Aptian time, rift extension gave way to broader, more uniform subsidence. Most of the existing production is from rocks deposited during the post-rift sag phase. Deposition was characterized by broad facies zones and distinctive faces differentiation. Subsidence rates declined from around 300m/my during rifting to 20 m/my during the Late Cretaceous, just before the onset of tectonic inversion related to opening of the Sea of Japan.

Genetic stratigraphic sequences in the Lower Cretaceous section of Songliao Basin, each bounded by a major lacustrine flooding surface, reflect a combination of external controls: tectonism, climate, lake level, and sediment supply. Cyclic changes in lake level corresponded in general terms with established Cretaceous sea levels, but with higher amplitude, more frequent variation typical of closed lake basins. Episodes of tectonically or climatically controlled clastic influx, particularly during times of reduced lake level when the lake shrank to 70,000 sq km or less, alternated with slow rates of fine-grained sedimentation during episodes of lake expansion to 230,000 sq km or more (Fig. 1). Compared to marine condensed sections, the lacustrine mudstone flooding intervals are anomalously thick, perhaps due to sediment sources which surrounded the entire depocenter.

Exploration has focused on deltaic reservoirs of the Aptian/Albian Qingshankou, Yaojia, and Nenjiang Formations, which contain more than 90 percent of known reserves. This genetic stratigraphic sequence is bounded below and above by thick, regionally extensive mudstone of the Qing-1 and Nen-1 Members, which originated in stratified, anoxic lakes, favoring preservation of immense volumes of oil-prone organic material (Fig. 1 ). Additional exploration targets are present beneath the deltaic interval, in lake-floor fans (identifiable on seismic) and channel-levee complexes of the Upper Jurassic rift section.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah