Characteristics and Genetic Analysis of the Deep-buried Weathered-crust Karst Hydrocarbon Reservoirs of the Lower Paleozoic Group in the Tarim Basin

摘要

The genetic analysis of the deep-buried reservoirs of the Lower Paleozoic carbonate rocks in the Tarimbasin is a difficult task involving many factors. Firstly, the object of study is carbonate rocks, which have undergone along term of modification. Secondly, the rocks are deeply buried with depths of 3800-7000 m in the Tarim basin. Theprimary reservoir properties formed in the deposition have been strongly modified during the deep burial process.Concurrently, the different burial depths in different areas result in diversities of burial temperature, pressure,underground water, hydrochemistry and various physicochemical changes, which further lead to differences in thediagenetic type, diagenetic property, diagenetic degree and their impacts on the reservoir properties. The LowerPaleozoic Cambrian and Ordovician carbonate reservoirs in the Tarim basin can be grouped into four types, i.e.,paleo-weathered-crust reservoirs, reef reservoirs, buried karst reservoirs and dolomite reservoirs. This paper presents adetailed discussion on the vertical and horizontal distribution characteristics, morphological division, reservoir propertiesand the efficiency in accumulating hydrocarbons of the paleo-weathered-crust type. Furthermore, its genesis is alsoanalyzed. We proposed that the composition of the carbonate rocks, the tectonic movement with associated fractures andfissures, the paleomorphology and paleoclimate, the sea level fluctuation, and the protection of the pores and fissures bythe deep burial diagenesis and burial dissolution are the main factors controlling the formation of thepaleo-weathered-crust reservoirs. We also consider that the petroleum exploration of the Lower Paleozoic carbonaterocks should be focused on the paleo-weathered-crust reservoirs.