Burial Dissolution of Ordovician Granule Limestone in the Tahe Oilfield of the Tarim Basin, NW China, and Its Geological Significance

With a comprehensive study on the petrology,geology and geochemistry of some Ordovician granule limestone samples in the Tahe Oilfield of the Tarim Basin,two stages of burial dissolution were put forward as an in-source dissolution and out-source dissolution based on macro-microcosmic petrology and geochemistry features.The main differences in the two stages are in the origin and moving pass of acid fluids.Geochemical evidence indicates that burial dissolution fluids might be ingredients of organic acids,CO_2 and H_2S associated with organic matter maturation and hydrocarbon decomposition,and the in-source fluid came from organic matter in the granule limestone itself,but the out-source was mainly from other argillaceous carbonate rocks far away.So, the forming of a burial dissolution reservoir resulted from both in-source and the out-source dissolutions.The granule limestone firstly formed unattached pinholes under in-source dissolution in situ,and afterwards suffered wider dissolution with out-source fluids moving along unconformities, seams,faults and associate fissures.The second stage was much more important,and the mineral composition in the stratum and heat convection of the fluid were also important in forming favorable reservoirs.