Diagenesis of Reservoir Rocks of Gaotaizi Oil Bed(Cretaceous) and Its Influence on Porosity Modification in Longhupao Field Area

Key words: 
• reservoir rocks /
• diagenesis /
• porosity modification

Abstract: Longhupao Field Area is located in the Songliao Basin. The Gaotaizi Oil Bed belongs to the second and third sections of Lower Cretaceous Qingshanqkou Formation in Longhupao Field Area. The reservoir rocks mainly deposited in distributary month bar and front thin sand sheet of lacustrine delta. The current geotemperature is 65 to 90 ℃, and the geothermal gradient is 3.9 ℃/100 m for Gaotaizi Oil Layer in Longhupao Field Area. Most reservoir rocks are lithic arkosic siltstone. The framework grains consist of 80% to 90% of reservoir rocks by volume. The framework composition is 20%～37% quartz, 23%～41% feldspar, and 14%～35% lithic rock fragments. Most rock fragments are of sedimentary and volcanic rocks. Sometimes carbonate particles are present. The carbonate particles, which is mainly ostracodes skeletons, are present in minor to trace amount, making up usually less than 5% of reservoir rocks. All clasticg rains are less than 0.5 mm in diam are believed to have. Compaction, cementation, dissolution, change of clay minerals, and displacement modified the sediments. The rocks are buried in 1 537～2 402 m. Compaction produled influence on sediments to a medium ou high degree because of fine clastic grain, poor sorting, some content of matrix and plastic carbonate particles. As a result of compaction, grain contacts are dominantly of long and concavo-convex types, and reduction of original pores is obvious. Compaction caused loss of 61.32% to 78.59%. Calcite is the most abundant authigenic minerals. Authigenic clays are rarely present Authigenic quartz is common, but present in trace amount. Authigenic feldspar is much less than authigenic quartz. Most calcite, among which clastic grains float, occurs in poikilotopic cement. The δ 13 and δ 18 O values of calcite cement range from 1.358‰ to 7.165‰ and -21.428‰ to -11.662‰, and average 2.958‰ and -19.351‰ (PDB), respectively. The δ 13 C and δ 18 O values of calcite cement change irregularly with depth, suggesting that burial diagenesis only slightly effect on calcite cement. The low values and narrow range of δ 13 C suggest that calcite cement formed early before maturation of organic material. Feldspar and intermediate/basic volcanic rock fragments are dissolved in different degree, forming various types of pores. Dissolution of quartz is often present, but weak, and only small dissolution pits are present in rims of quartz grains. Dissolution of calcite is also weak, and occurs only in cement of ostracoda coelomata. Illite is main clay mineral, and chlorite is present only in minor amount. Kaolinite basically disappeared. Smectite mostly changed to illite or mixed-layered illite/smectite. Vitrinite reflectance (Ro) rages from 0.61% to 1.39%, suggesting that organic material has already matured. Ancient geotemperature is higher than current geotemperature. Diagenesis proceeded to middle late stage. Compaction and cementation are the principle processes contributing to reduction of porosity. Dissolution of feldspar and intermediate/basic volcanic rock fragments made porosity increase to some degree. Compaction and calcite cementation prevented growth of authigenic quartz and formation of secondary porosity zones. Partial preservation of original pores and development of feldspar dissolution are due to framework consisting of clastic grains and calcite cement of early precipitation.