Study On Petroleum Accumulation Rules Of Shiwu Fault Depression In Southern Songliao Basin

Shiwu Fault Depression is a superposed terrestrial oil-bearing basin composed of the underlying fault-subsidence and the overlying depression. It is located in the southeastern edge of the Southeast Uplift in Songliao Basin. The paper study the tectonic evolution, structure features and reverse structural zones of Shiwu Fault Depression in detail. The hydrocarbon accumulation assemblages were classified based on the theory of petroleum system. The organic faces model of the depression was constructed based on the integrated study of depositional environment, the origin of source rock and the organic matter types. The reservoir types, their distribution as well as hydrocarbon migration and accumulation features were analyzed based on the representative sections. The hydrocarbon accumulation periods and the dynamic accumulating processes were determined based on reservoir fluid inclusions tests and the hydrocarbon generation history as well as tectonic evolution. The reservoir accumulation model was built based on these studies. Finally, this paper analyzed the major control factors for hydrocarbon accumulation. Based on these studies, the remarkable achievements and understandings of this study are summarized as follows:1. Shiwu Fault Depression tectonic evolution can be divided into four stages, namely, the crust uplifting stage before fault-depression and volcano eruption stage at the beginning of fault-depression; fault-depression stage; subsidence stage and the shrinking stage of basin uplifting. The general character of the fault-depression is an asymmetric dustpan-like depression, with faulting in the west and overlying in the east. But the deep structure is very complicated where the multiple small scale half-graben fault-depressions were developed.2. The whole Shiwu Fault Depression can be classified as an independent petroleum system. The stratigraphy can be vertically divided into three sets of hydrocarbon accumulation assemblages - lower hydrocarbon accumulation assemblage, middle accumulation assemblage and upper hydrocarbon accumulation assemblage. The lower one can be further divided into in situ type and upper-generation- lower-accumulation type.3. The major source rocks are the Shahezi Formation and the Yingcheng Formation. Controlled by tectonic evolution and burial history, the deep depression formations close to Sangshutan Fault and the source rocks in several small scale fault depressions formed at the initial stage of fault-depression are over-matured. To the east, the burial depth becomes shallower. the source rocks are in mature stage.4. The lower parts of the reservoir in Shiwu Fault Depression are tight reservoir with low porosity and permeability. The shale of the second member of Quantou Formation acts as regional cap rocks. The shale deposited during fault-depression stage is local cap rocks. The horizontally discontinuous distribution of shale formation controls the distribution of reservoir.5. Reservoir types include the structural, the stratigraphical and the structural-stratigraphical, themajority reservoirs are stratigraphical type. Vertically there are two hydrocarbon accumulation zones, and horizontally the hydrocarbon accumulation zones show ring-shaped feature. The major unconformities are important gas accumulation areas. The hydrocarbon was accumulated by vertical migration and lateral migration respectively.6. This area has three major hydrocarbon accumulation periods, that is, the end of the Dengluoku stage, Quantou-Nenjiang stage and the end of Nenjiang stage. The end of Dengluoku stage and the Nenjiang stage are the key periods of hydrocarbon accumulation for the petroleum system. Structure is the control factor for hydrocarbon accumulation.7. The central structural belt, the western steep slope belts, and then southeastern slope belt in Shiwu Fault Depression are favorable areas for hydrocarbon accumulation.