Structural Characteristics And Hydrocarbon Migration And Accumulation Of Shanle Oil Field, Tuha Basin

The Shanle oil field of the Tuha basin is located in the tectonic zone of the front of the Bogda mountain, the geological condition of which is complex, and the sand layer and coal layer are developed interactively. The Shanle oil field is coal reservoir featured in extra-low porosity and extra-low permeability. Although the production increased yearly, there were no major breakthrough in the exploration and development in recent years, which was the result of the complex geological background, preliminary imperfect structural analysis, no enough in-depth studies of the effect of structure and fracture on the hydrocarbon accumulation, the rapid changed source direction and the multiple source-reservoir-cap assemblage. To solve the above problems, based on the seismic, well logging and drilling data and combined the geological conditions of the study area, this paper recognized and analyzed the structural characteristics of the Shanle oil field using Landmark workstation, on this basis, the rules of hydrocarbon accumulation of the structural reservoir in the typical blocks were summarized, the mode of hydrocarbon accumulation was established, the favorable hydrocarbon trap was predicted, and finally, the favorable areas for further exploration and development were indicated. The results show that:(1) The Shanle area mainly developed three types of fracture: basement-involved fault, seal-detachment fault and tear fault. The basement-involved thrust faults in the both sides of the study area formed in the Late Indosinian, and were active in the late period. The seal-detachment faults in the central of the study area formed in the second-stage of the Yanshan movement, while in the third-stage of the Yanshan movement, the tear faults formed and cut the seal-detachment faults formed in the earlier period.(2) In the Shanle area, faults are the important factors for hydrocarbon accumulation. During the period of forming the basement fault, a plenty of structural cracks developed, which communicated the source rocks deposited in the Lower-Middle Jurassic and Triassic formations, and provided the favorable channel for the upward or lateral migration of hydrocarbon. The associated structural cracks in the period of the seal-detachment faults formation were mostly filled by calcite, silica and other minerals, which was beneficial for the preservation of the early generated hydrocarbon. However, affected by the late Yanshan movement and Himalayan movement, these faults formed in the earlier period activated and the fault surface staggered, which caused the hydrocarbon loss of the earlier reservoirs and accumulated in the favorable traps of the shallow layers.(3) The reservoir types of the Shanle area can be divided into structural reservoir and lithological reservoir, which were subdivided into five subclasses: faulted anticline type, faulted block type, faulted nose type, upward thinning out sand body type and lens-shaped sand body type.(4) The hydrocarbon accumulation rules of the structural reservoirs in the typical blocks are:â‘ close to the basement-involved faults that could communicate the source rocks;â‘¡the trap morphology is controlled by both structure and faults;â‘¢the lower of the effective reservoir mainly develops unfilled high-angle cracks and the upper of that mainly develops filled low-angle cracks.(5) The hydrocarbon traps of the faulted nose and lithological types were predicted and the favorable areas for further exploration and development were pointed out. The sealing condition of the footwall fault of the basement-involved thrust fault in the south of the study area is good, which is easy to form faulted nose reservoir in the first and second members of the Xishanyao formation. In the second member of the Xiashanyao formation in the middle-west of the study area, the foreset sand body of delta facies is likely to form the upward thinning out sand body reservoir by the result of the later tectonic movements, while the turbidite sand body of lacustrine facies deposited in the third and fourth member of the Xishanyao formation is prone to form the lens-shaped sand body reservoir.