Evaluation And Study Of Deep Natural Gas Cap Rock In Xujiaweizi Depression

The important producing gas formations are K1yc1, K1yc3 and K1yc4, and there are small-scale gas reservoirs in base, K1d2 ,K1d3, K1d4 and K1q1, but the cap rock categories of sealing the natural gas are unclear and the importance and sealing ability differences of the various sets of cap rock are not very clear. Carried out for this problem the following is researched in the paper. First, by use of drilling, logging and analysis data, combined with gas reservoir profile, it is concluded that there are 3 types of cap rocks in Xujiaweizi fault depression, that is, mudstone, volcanic rock and argillaceous conglomerate. By analyzing, statistics and contradistinction the identification mark and distribution of caprock of deep gas in Xujiaweizi fault depression, it is considered that there are 5 sets of regional caprocks ( K1q2, K1q1, K1d4, K1d3 and K1d2 ), a local cap rock on top of K1yc1 and interlayers in K1yc1 and K1yc3. The regional caprocks(K1q2, K1q1 and K1d2)are large-area distribution, large thickness of mudstone accumulated and high-mud ratio; The local cap rock on top of K1yc1which is only distributed in the southern region, the thickness of thin and high-similar mud ratio; Interlayers in K1yc1 and K1yc3 distribution range is small and the thickness of thin in Xujiaweizi fault depression. Second these types of micro-sealing ability of caprock is not the same, the micro-sealing ability of 5 sets of regional caprocks are the strongest, presenting gradual weakening from bottom to top, a followed by the caprock on the top of K1yc1, the worst is interlayers in K1yc1 and K1yc3. While by used of cap rock thickness and fault perpendicular distance of wearing each set of cap rock informations, analysis of the each set of cap rock has been broken damage by fault, and the result showed that K1q2, K1q1 and K1d2 cap rocks damage level is a lesser extent, a larger extent of the damage cap rock on top of K1yc1. Third, though the analysis of many factors affecting the natural gas caprock, cap rock thickness, displacement pressure, depth, extent of fault damage cap rock, pressure coefficient and gas viscosity affect cap rock sealing ability, so use of these six parameters to establish a set of caprock sealing ability of the comprehensive evaluation method, and established a set of six parameters caprock sealing ability of the comprehensive evaluation method, as well as the regional cap rocks and the local caprock on the top of K1yc1 were evaluated. Evaluation results show that comprehensive evaluation ability of natural gas cap rocks of K1q1+2 and K1d2 is powerful and mainly better and good level, as well as the cap rock on the top of K1yc1 is weak and mainly in poor level. Last, through the relationship study between the cap rock and the distribution of natural gas:①Since natural gas accumulation grade is low in reservoir bed, so the poor cap rock on the top of K1yc1 still seal natural gas.②The lower K1d2 cap rock and cap rock on the top of K1yc1 controlled natural gas distribution under the two sets of cap rock vertically.③Difference of reservoir-cap rock combination leads natural gas to accumulate primarily in the southern of the fault depression.④Compared to lower interval transit time volcanic cap rock, natural gas primarily accumulates under the stronger sealing high acoustic interval transit time volcanic rock volcanic cap rock.⑤The gas reservoir cap rock sealing ability is strong, so the cap rocks are able to seal natural gas in reservoirs.