渤海湾盆地东濮凹陷膏盐岩与油气的关系

Relationship between Gypsum-salt Rock and Oil-gas in Dongpu Depression of Bohai Gulf Basin

从东濮凹陷蒸发岩系的地质特征出发,系统分析了膏盐岩对油气成藏诸要素的影响及其对油气分布的控制作用。研究结果表明,东濮凹陷烃源岩与膏盐岩共生,膏盐岩—砂泥岩过渡带烃源岩有机质丰度最高;膏盐岩发育区烃源岩有机质类型以Ⅱ1型和Ⅰ型为主;膏盐岩的高热导作用使盐上烃源岩生烃门限深度降低和盐下烃源岩过成熟门限深度增加,有效地扩大了生烃窗范围。膏盐层下部砂岩储层孔隙度相对正常压实地层而言整体偏大,且具有随深度增加表现为先增加后减小,孔隙度最大值出现在膏盐层下部一定距离内。膏盐层不仅对油气具有很强的封堵能力,而且易于形成盐下、盐间、盐上及盐岩边缘等多种圈闭类型。膏盐层厚值区形成的异常压力明显大于膏盐层边缘区,油气更易于在膏盐层的遮挡下向膏盐层边缘区运移。综合分析认为,东濮凹陷膏盐层边缘区不仅具有良好的供烃和圈闭条件,而且具备油气充注的通道和动力条件,是最有利的油气聚集区。

Dongpu depression is located in southeastern Bohai Bay Basin, five sets ot gypsum-salt rock clevelopect, and the maximum cumulative thickness can be up to 950 m. The lithologic variation of gypsum-salt layer can be divid- ed into three zones including gypsum-salt zone, gypsum-salt and sand-mudstone transition zone, sand-mudstone zone. Exploration practice confirmed that gypsum-sah rock on hydrocarbon enrichment in vertical or in horizontal has an ira-portant controlling role, performing for nearly 93.7 % of oil and nearly 80% of gas in Dongpu depression distribution in the north part, and most oil and gas distributes among gypsum-sah layers. Controls of gypsum-sah rock on pool forming elements were systematically analyzed, and the results show that the source rock in gypsum-sah and sand-mudstone transition zone has the highest organic matter abundance because of its advantage for high quality source rock accumulation and preservation, the next is that in gypsum-sah zone, and that in sand-mudstone zone have relative lower organic matter abundance. Meanwhile, the farther source rock away from gyp- sum-salt rock, the lower organic matter abundance it has, and when the distance between source rock and gypsum-sah rock exceeds 30 or 40 meters, the influence on organic matter abundance by gypsum-sah rock decreases significantly. For high water salinity, many halophilic bacteria and algae adapted to high salt environments thrive and become a ma- jor source of sedimentary organic matter, which is conducive to the formation of sapropelic organic matter. For the gyp- sum-sah rock has high thermal conductivity and low heat generation rate, it can transmit ground temperature easily from deep stratum to shallow stratum, which could result in source rock in the upper part of gypsum-sah stratum has lower temperature gradient and abnormal high temperature than that of no gypsum-sah stratum, to the contrary, source rock in the lower part of gypsum-sah stratum has higher temperature gradient and abnormal low temperature than that of no gypsum-sah stratum. The anomal temperatures in the upper and lower part of gypsum-sah stratum result in reduc- ing the generation threshold depth and increasing the postmaturity threshold depth of source rocks, this can effectively expand the scope of hydrocarbon generation window. The relationship between reservoir porosity and its depth shows that the porosity of reservoir covered by gypsum- salt rock has 2-10 percent larger than that of normal sediment stratums, it firstly increases and then decreases with increasing depth, and the maximum porosity appears within a certain distance from gypsum-sah rock. Statistical results on the distribution of gypsum-sah rock and the height of hydrocarbon bearing strata show that most reservoirs in gyp- sum-sah area distribute below 0- 50 meters thickness of gypsum-sah strata, only 50 meters thickness gypsum-salt stra- ta could seal 500 meters height of hydrocarbon, and gypsum-sah layers plugging maximum oil height is to be signifi- cantly greater than that of gas. Gypsum-sah rock has strong plasticity, which is apt to form kinds of traps under syp- sum-sah, inter sypsum-sah, upper sypsum-sah and at the edge of gypsum-salt. Under the similar structural conditions, the abnormal formation presssure under the thicker gypsum-sah rock is significantly greater than that at the edge of gypsum-sah rock, so the oil and gas is easier to be filled and preserved in the traps at the edge of gypsum-sah rock. Even there has oil and gas migrating towards gypsum-salt rocks, mostly will be affected by abnormal pressure at the edge of gypsum-sah rock and accumulated. The strong vertical packer role of gypsum-sah rock on oil and gas is conducive to its lateral migration in the sandbody below gypsum-salt layer. Synthetical analysis lead to that hydrocarbon in northern Dongpu depression shows ring or semi-ring shaped distri- bution around the thickness center of gypsum-sah layer. At the edge area of gypsum-sah rock not only has good source rock and good trap conditions, but also has proper hydrocarbon charging pathway and dynamics, so it becomes the most favourable area for oil and gas accumulation.