致密碳酸盐岩盖层封闭机理与评价方法

Tight mud-bearing and pure carbonates, as the direct caprock of large oil and gas fields on the northern slope of Tazhong uplift in the karst reservoirs of Yinshan formation, is an important control factor to lead the oil, gas, and water to distribute complexly. The proposed key scientific problem is the sealing mechanism of the Paleozoic tight carbonate. That is why the tight carbonate rocks with normally low value of displacement pressure could seal well, and how the Paleozoic tight carbonate caprock is formed in the geologic history. There are four aspects in connection with the tight carbonate caprock: the characteristics of rock structure, the key influence factors of sealing, the comparison of adjacent reservoirs’ displacement pressure, and the evaluation of parameters and methods. The major technical methods includes the observation of cores and rock slices, the analysis of mineral composition, the data interpretation in geophysical logging (natural gamma, bilateral resistivity, imaging), the test of physical property and breakthrough pressure, and the detection of fluid inclusions’ organic components. This research has two targets: the first one is to primarily answer the tight carbonate caprock sealing oil and gas mechanism and boundary conditions, which provides a reasonable explanation in low displacement pressure are able to seal the distribution of hundred million tons of oil and gas reservoir groups; the second is to prioritize appropriately key parameters and applicable boundary conditions for Paleozoic carbonate caprock evaluation in China superimposed basins, to form a highly feasible evaluation approach and endow a predictive function.

含泥和纯的致密碳酸盐岩作为塔中北斜坡鹰山组岩溶储集体大油气田群的直接盖层，是导致油气水复杂分布的重要控制因素。拟解决的关键科学问题是古生界致密碳酸盐岩封闭机理，即排替压力值普遍不高的致密碳酸盐岩为何能起到很好的封闭作用以及地质历史时期古生界致密碳酸盐岩盖层是如何形成的。针对致密碳酸盐岩盖层开展岩石结构特征、封闭性关键影响因素、与相邻储集层排替压力比较和评价参数与方法四方面研究。主要技术手段包括岩心及岩石薄片观察、矿物成分分析、地球物理测井（自然伽马、双侧向电阻率、成像）资料解释、物性测试、突破压力测试、流体包裹体有机组分检测。研究目标有二，一是初步回答致密碳酸盐岩盖层封闭油气的机理和边界条件，合理解释排替压力不高却能封闭连片分布的亿吨级的油气田群；二是优选适合我国叠合盆地古老碳酸盐岩盖层评价的关键参数及适用的边界条件，形成操作性强的评价方法并赋予其预测功能。

塔中隆起奥陶系良里塔格组不整合位于鹰山组之上，紧邻不整合面含油气层的直接盖层是良3-5段泥灰岩，鹰山组内幕油气层的直接盖层是鹰山组内叠加分布的致密碳酸盐岩层（高阻层）。勘探结果显示油气水分布复杂，出现“上水下油气”现象。随着评价井、开发井的跟进，发现碳酸盐岩盖层是导致油气水分布复杂性的重要原因。碳酸盐岩盖层样品排替压力值普遍不高，为何起到很好的封闭作用？在地质历史时期古碳酸盐岩盖层如何形成？本项目针对核心科学问题，设置主要研究内容为：致密碳酸盐岩盖层的岩石结构特征研究（包括盖层样品岩石结构分析；测井曲线以及成像测井资料解释；通过测井曲线计算获得孔隙度渗透率参数）；致密碳酸盐岩盖层封闭性关键影响因素研究（包括泥质含量、盖层厚度、排替压力等）；致密碳酸盐岩盖层与相邻储集层排替压力比较研究（包括排替压力的测试及计算；致密碳酸盐岩盖层横向连续性；致密碳酸盐岩盖层与下伏储层排替压力差分析）；致密碳酸盐岩盖层评价参数与方法研究（包括各项参数对盖层封闭能力敏感性及建立评价方法；应用探井及评价井资料预测有效封闭区）。通过4年的研究，明确了良里塔格组泥灰岩盖层封闭性受泥质含量、排替压力以及盖层厚度的影响，并结合油气勘探提出盖层封闭性综合评价指标；鹰山组致密灰岩主要通过高阻层和下伏储集层的排替压力差封闭油气。本项研究成果不仅在塔中隆起中古43区块奥陶系海相碳酸盐岩滚动勘探开发中，高产富集区优选、井位部署发挥了指导作用，同时，对于塔北隆起、古城低隆以及柯坪地区的碳酸盐岩盖层封闭性及其对油气勘探影响的研究也起到很好的借鉴作用。其科学意义就在于，通过分析碳酸盐岩盖层封闭性关键控制因素，厘定了盖层封闭性评价参数，建立封闭性评价方法，合理阐述了塔中隆起排替压力较低的致密碳酸盐岩盖层封闭机理，拓展了油气盖层的研究，丰富了海相油气地质理论。