Unconformities and their influence on lower Paleozoic petroleum reservoir development in the Tarim Basin

Abstract

Abstract Ten regional unconformities punctuate the Lower Paleozoic carbonate succession of Tarim Basin. They can be divided into three orders and include four unconformity types. According to their extents and intensities of development, they can be categorized into three orders, a first-order unconformity of great lateral extent and depth of erosion, and second-order and third-order unconformities of lesser extent and erosional intensity. From the viewpoint of their origins, these unconformities can be classified into four types, structural unconformities, long-term exposure unconformities, synchronous sedimentary exposure unconformities and superimposed unconformities that include combinations of the previous three types. Through analyses of the geographic distribution of unconformity types across the geographic extents of the 10 unconformities and their temporal evolution, it was determined that unconformities of various orders have different effects on the development of carbonate reservoir beneath these unconformities. First-order and second-order unconformities are prominent in the development of karst reservoir of the Tarim Basin. Periodic exposure of carbonate rocks which occurs during erosion that generated third-order unconformities contributed to the development of multiple karst intervals. In addition, the abundance and size of karst intervals in progradational strata increased during sea level falls. Origins of unconformities control scales and types of karst reservoirs. Tectonic (angular) unconformities developed during tectonic uplift and denudation, which controlled the development of weathering crust karst reservoirs within zones containing buried-hill (e.g. low paleo-uplift). Long-term exposure (parallel) unconformities formed during long-term sea level fall which controlled the development of regional interlayer karst reservoirs within the platform. Synchronous sedimentary exposure unconformities formed due to intermittent exposure caused by sedimentary basinal infilling and subdued sea level fluctuations which controlled the development of penecontemporaneous karst reservoirs of smaller geographic extent along platform margins. Superimposed unconformities formed due to multiple superposed tectonic movements, which caused the superposition of multiple weathering crusts as individual karst reservoirs, or the stacking of karst reservoirs developed beneath different unconformity types.