Differential hydrocarbon accumulation model of the Ordovician in Tazhong Ⅱ block, Tarim Basin

The study aims to analyze the nature, distribution patterns and origin of fluids in the Ordovician by means of reservoir geochemistry, in view of existing problems concerned with coexistence of multiple phases of oil and gas, large difference of productivity and limited knowledge of hydrocarbon enrichment modes of the Ordovician reservoirs in Tazhong Ⅱ block, Tarim Basin. The pressure-volume-temperature (pVT) analysis of the Ordovician reservoir fluids shows that condensate gas reservoirs and light oil reservoirs coexist along strike-slip faults. The crude oil maturity parameters indicate the oil is product of mature stage, and the carbon isotope and hydrocarbon composition ratio of natural gases reveals that the gas is a mixture of the gas associated with the crude oil and oil-cracking gas. The analysis of diamondoid compounds and natural gas shows that the charging of gas from cracking of oil in reservoirs below the Middle Cambrian evaporite layer is an important factor leading to the formation of the Ordovician condensate gas reservoirs. The analysis in combination with 3D seismic interpretation data of high precision indicates that the hydrocarbon distribution is controlled by strike-slip faults, and that the crude oil density and wax content are relatively lower, and gas/oil ratio, drying coefficient and 4-MDBT/1-MDBT ratio are relatively higher in horsetail-type graben, wing tail-type graben and intersection of strike-slip fault and thrust fault which are of favorable channels for hydrocarbon charging. The coupling of the periodical opening of strike-slip faults with the sealing mechanism of evaporite intervals controls the hydrocarbon accumulation of the Ordovician in the study area. It is considered that the hydrocarbon reservoirs in Tazhong Ⅱ block are characterized by juxtaposition of source rock and reservoir, vertical migration and enrichment along strike-slip faults, and multi-stage scattered hydrocarbon charging, all of which are an important reason for the differential hydrocarbon distribution in the Ordovician reservoirs. Strike-slip faults control hydrocarbon accumulation in the Ordovician, and there is great exploration potential in the horsetail-type graben, wing-tail-type graben, and intersection of strike-slip faults and thrust faults. Large-scale gas reservoirs may exist in sequences below the Middle Cambrian evaporite layer, and this is in need of further attention.