Abstract:

In order to reveal the control functions of Boerjianghaizi fault on natural gas accumulation of Upper Paleozoic in Hangjinqi block of the northern Ordos Basin, the history of the fault activity, the lateral and vertical sealing ability of the fault were studied by using fault growth index evolution, fault shale gouge ratio method and fault connectivity probability method through seismic and drilling data of this area. Then Boerjianghaizi fault effects on the natural gas enrichment zones were discussed. The results showed that Boerjianghaizi fault had three obvious peaks of activity in the history of evolution, which were Caledonian to Early Hercynian fracture period, Indosinian to Early Yanshanian squeezing thrust fault activity period, and Mid-late-Yanshanian strike-slip tearing activity period. Mid-late-Yanshanian strike-slip tearing activity period matched well with the main stage of gas accumulation of Upper Paleozoic. It was pointed out that sealing ability of Boerjianghaizi fault had the characteristics of “horizontal sectioning, vertical layering”. In the horizontal direction, Boerjianghaizi faults were divided into lateral migration fault areas and lateral sealed fault areas according to the shale gouge ratio (SGR) which equals to 0.3 as the dividing line. In the vertical direction , the fault plane was vertically sealed in Upper Shihezi and Shiqianfeng formations because the fault sealing coefficients were almost less than 1.0 in the section, while the fault plane was vertically open in Lower Shihezi and Shanxi formations because the fault sealing coefficients mostly range from 4.0 to 10.0 in the section. Meanwhile, in the lateral migration fault areas, natural gas generated from the main source areas which were in the south side of Boerjianghaizi fault was transmitted in the vertical and lateral direction through the fault, then natural gas accumulated in large amount in the north side of the fault easily, but in the lateral sealed fault areas, it is easy to find the natural gas pools in the south side of the fault.

Key words: Ordos Basin, Hangjinqi block, Boerjianghaizi fault, fault sealing property, natural gas accumulation