鄂尔多斯盆地镇泾地区长8致密储层成藏期临界物性厘定

鄂尔多斯盆地三叠系延长组广泛发育致密砂岩油气藏，其中长8油层组是该区富有潜力的勘探开发层位。成岩作用的复杂性，使得长8致密储层现今孔隙度与成藏期相比发生很大变化，因此，对长8油层组成藏充注期储层临界物性的厘定，有助于动态地评价储层质量，客观地评价储层的含油气性。根据粒度分析资料、砂岩薄片资料及含油产状统计，确定了研究区有效储层岩性下限为油斑—油迹细砂岩。并通过长8油层组有效储层的测井解释、试油、常规物性、压汞、核磁共振、相渗透率、覆压孔渗及应力敏感性试验等资料，基于统计学和超低渗透油藏流体渗流机理，优选合适的评价方法求出取镇泾区块长8油层组有效储层的物性下限为7.17%和0.10 ×10-3 μm2。基于孔隙度与时间、温度、深度、压力的函数关系，以地史模拟为基础，利用初始孔隙度及现今实测孔隙度进行端点约束，利用时深指数TDI对致密储层的临界物性进行厘定，得出两期油气充注的临界物性约为27.0%和17.0%，表明自油气充注以后储层发生了较严重的致密化过程。

Determination of Lower Limits of Critical Properties of Chang 8 Tight Sandstone Reservoirs,Zhenjing Block,Ordos Basin

Tight sandstone reservoirs occur widely throughout the Triassic Yanchang Formation in the Ordos Basin. The Chang 8 oil-bearing layer is a potential exploration and development horizon in the area. Complex destructive diagenesis further compacted the reservoir, such that present-day porosity differs considerably from what is usual for the accumulation period. Therefore, determining its critical physical properties during knowledge of the charging periods helps to dynamically evaluate the quality of the reservoirs and their oil and gas properties. Firstly, by combining grain-size analysis, sandstone sheet data and comprehensive statistics of the oil grade, the lower limits of the lithology in the area were determined from spot-oil traces in the fine sandstone. Then, the log of the Chang 8 oil-bearing sandstone and oil production tests was analyzed by conventional physical property testing, mercury injection analysis, nuclear magnetic resonance (NMR) testing, two-phase permeability analysis, overburden pressure and stress sensitivity analysis, etc. Based on statistical principles and fluid seepage mechanisms of ultra-low-permeability reservoirs, several methods applicable to tight reservoirs were selected to comprehensively determine the lower limits of the physical properties of effective reservoirs in the Chang 8 oil-bearing layer. Finally, the initial porosity was reconstructed by geohistory modeling from the relationships between porosity and time, temperature, depth and pressure. The measured porosity and the critical physical properties of the reservoirs are reported in terms of a time-depth index. The preferred NMR method of comprehensively determining the lower physical limits, along with displacement pressure testing and oil-bearing occurrence methods appropriate to tight reservoirs, gave a porosity of 7.17% and seepage of 0.1×10–3 μm2. The geohistory model endpoint constraints were the original (pre-compaction) and measured porosities. The time-depth index showed that the critical porosities during the two periods of hydrocarbon charging were about 27.0% and 17.0%, indicating that the reservoirs were significantly compacted after hydrocarbon charging.