塔河油田奥陶系古岩溶地表河和地下河沉积和地球化学特征

Sedimentary and geochemical characteristics of sandstones and mudstones deposited in surface rivers and subsurface rivers in Ordovician karsted carbonates,Tahe Oilfield

塔河油田奥陶系发育了典型的岩溶储层,大于70%的缝洞空间在岩溶发育期及岩溶埋藏期被沉积充填物、垮塌充填物和化学充填物所充填,造成岩溶储层极度非均质性。通过岩心观察、测井资料解释、岩矿和地球化学分析,建立缝洞充填物识别方法。结果表明:沉积砂泥岩占总缝洞充填物的60%以上,地表河和地下河沉积特征明显,其中沉积砂岩是重要的油气储层;两种河流均发育心滩和河漫滩,前者砂岩颗粒较粗,常为含砾的粗—中砂岩,地化分析表明为氧化环境产物;后者以细砂岩为主,石英含量比前者高40%,为弱氧化—弱还原水体沉积;地表河沉积砂泥岩常与坡积物互层,地下河沉积砂岩与垮塌角砾岩或潜流带细粒沉积物呈互层;前者受压实作用明显,储集物性差;后者压实作用很弱,物性好、含油性好;地下河沉积砂岩是径流岩溶带重要的储层,对于深入认识塔河油田岩溶储层地质特征具有重要意义。

Ordovician carbonates in Tahe Oilfield were typical karst reservoirs in the world. Fracture-caves were general storage spaces for oil and gas in the karst carbonates, but the spaces more than 70% had been filled with river deposits, collapsed breccias and chemical materials, which resulted in heavy heterogeneity in the karst reservoirs. By investigating drilling cores and well-log-ging and by using rock-mineral and geochemical measurements on some core samples, the authors found that sedimentary sands and muds were over 60% in the total fracture-cave fillings;the sediments were obviously in sequences of surface rivers and subsurface rivers which were developed in the carbonates during the karst on the Tabei uprise;and the sandstones deposited in the subsurface rivers were important reservoirs in caves of runoff karst zone. Channel bars and river banks are identified from the sediments depos-ited in either surface rivers or subsurface rivers. The sandstones deposited in surface rivers are usually in coarse-to-medium grain size, but in the subsurface rivers they are characterized as fine-sands or silt-sands. Composition and texture maturities of the sub-surface-river sandstones are much higher than those of surface-river sandstones:the quarts contents in the subsurface-river sand-stones is 40% higher than those in surface-river sandstones. The surface-river sandstones were deposited in open oxidative water, and interbedded with slope wash breccias;in comparison the subsurface-river sandstones were deposited in weak-oxidative to weak reductive waters, and interbedded with collapsed breccias, which were in good physical condition for oil and gas. The subsurface-river sandstones are good reservoirs in runoff karst zones in Tahe Oilfield. Oil production from the sandstones is expected to in-crease the oil-recovery in the karst reservoirs.