Diagenesis Of The Clastic Reservoir Of Kongdian Formation In The Northern Sag Of Weibei Depression

Natural gas is fairly rich in the northern sag of Weibei depression. It has not been well developed until now. On the basis of researches in the past, this dissertation studies diagenesis of the clastic reservoir of Kongdian formation by describing drilling cores, taking and analyzing rock samples in order to understand how the characters of reservoir change during burial process and help to find profitable reservoir. The following conclusions are reached after the study: 1 Diagenetic events occurred in the clastic reservoir rocks during burial diagenesis include compaction and pressure solution, cementation and replacement of carbonate, cementation of quartz, cementation and transition of kaolinite, illite/smectite mixed-layer clay minerals and chlorite, overgrowth of playgioclase, cementation of pyrite and recrystallization of carbonate cements and clay minerals in the matrix. The main diagenetic events such as compaction, cementation of calcite, quartz, ferroan calcite, ferroan dolomite started successively.The main factors affecting diagenesis are the original components of sediments and sedimentary facies, changes of geotemperature and geopressure with depth increasing, pH values and other chemical characters of formation water. The clastic reservoir of Kongdian formation in the sag are in different diagenetic periods, from period B of early diagenetic stage to period B of late diagenetic stage, according to the depth. 2 Diagenetic minerals formed during burial diagenesis include calcite, ferroan calcite, ferroan dolomite, quartz, kaolinite, R0I/S, R1I/S, R3I/S, diabantite, plagioclase and pyrite. As diagenetic intensity increased, the Fe, Mg contents of carbonate cements and Fe, Alâ…£contents of diabansite increased, the Si content of diabansite decreased and ilite/smectite mixed-layer mineral became more ordered. Origin of the material needed for formation of carbonate cements of the early and late diagenetic stages are different. 3 The paleogeothermal gradiant values are greater than that of today. 4 As diagenetic intensity increased with increasing depth, the reservoir changed from the conventional type to the tight one at 3000m in depth. 5 The main diagenetic events lealing to the reservoir becoming tight include compaction and cementation of carbonate. The porosity and permeability of the reservoir were more notably influnced by compaction above 2800m in depth, and cementation of carbonate became the leading role in the deeper place. 6 Pore types of the reservoir include fissures, intergranular pore, innergranular pore and that among interstitial materials. Pore of reservoir is universally very small. Fissures were caused by tectonic movements as the reservoir became more fragile with diagenetic intensity increased.