塔里木盆地塔中Ⅱ区奥陶系油气差异富集模式

Differential hydrocarbon accumulation model of the Ordovician in TazhongⅡⅡblock,Tarim Basin

针对塔里木盆地塔中Ⅱ区奥陶系油气多相态共存、产能差异和富集模式认识不清的问题,采用油藏地球化学方法分析了奥陶系流体性质、分布特征及成因。油藏pVT相态分析认为存在凝析气藏与轻质油藏沿走滑断裂共存的现象,原油的成熟度参数显示为成熟阶段的产物,天然气碳同位素和烃类比值揭示组分为原油伴生气和原油裂解气的混合,金刚烷化合物及天然气分析显示中寒武统盐下原油裂解气的充注是奥陶系凝析气藏形成的重要原因。结合高精度三维地震解释发现油气分布受到走滑断裂的控制,马尾、翼尾、走滑断裂带与逆冲断裂交汇部位具有较低的原油密度和含蜡量,以及较高的气/油比、干燥系数、4-MDBT/1-MDBT比值等,为油气充注的有利通道,走滑断裂周期性开启与膏盐层封堵机制耦合控制了塔中Ⅱ区奥陶系油气成藏。综合研究认为,塔中Ⅱ区油气具有侧生邻储、走滑断裂垂向输导、沿走滑断裂富集的特征,多期点状油气充注是造成奥陶系油气差异分布的重要原因。走滑断裂控制了奥陶系油气富集,马尾地堑、翼尾地堑、走滑断裂带与逆冲断裂交汇构造部位仍有较大的勘探潜力,中寒武统膏盐层下可能发育大规模气藏,需要进一步关注。

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 TazhongⅡblock,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 TazhongⅡblock 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.