A conceptual model to investigate the impact of diagenesis and residual bitumen on the characteristics of Ordovician carbonate cap rock from Tarim Basin, China

摘要

The Lower-Middle Ordovician Yingshan (O1-2 y) Formation associated with large unconformity weathering crust was the main hydrocarbon reservoir in the Tarim Basin, and the dense limestone/dolomite within this formation was revealed as the local carbonate cap rock. Based on petrophysical properties, thin sections, cathodeluminescence (CL) and scanning electron microscope (SEM) measurements, the effects of cap rock on depositional settings, diagenesis and pyrobitumen were carefully analyzed. Carbonate cap rock dominated by micrite has ultralow porosity and permeability, with an average porosity of 0.568% and permeability of 0.07 x 10(-3) mu m There is a significant bottom-up thickness variation (from several to tens of meters) in different members within the O1-2 y formation. The steady cap rock is more likely to be distributed in low-energy depositional environments, corresponding to intershoal sea, low-lying land of open platform and lagoon of restricted platform. Constructive diagenesis processes mainly comprise extreme filling, compaction and cementation, resulting in a significant decrease in the quantity and size of pores. Conversely, leaching and paleokarst dissolution, together with diagenesis associated with hydrothermal fluids, has a detrimental effect on the effective preservation of cap rock. Three types were summarized to characterize the cementation-compaction relationship between cement distribution and intraclast deformation. The increase in intraclast size indicate that the intergranular pores are likely to be cemented. A good sealing impact on the underlying oil-gas is confirmed in sparry calcarenite when its cement content reaches 15% and higher by volume. Early compaction vs. early cementation processes probably affect the sealing capacity, so high quality cap rock is primarily presented while cementation precedes compaction. Highly abundant stylolites filled with residual bitumen and precipitated cements in the cap rock intervals consistently have low poroperm values, resulting in micropore occlusion and occlusion of the late hydrocarbon migration pathways, which can act as barriers for the upward migration of oil-gas. A conceptual model of the evolution feature of cap rock provides a plausible interpretation for better understanding of the spatial distribution of carbonate cap rock in future exploration and development