准噶尔盆地腹部超压顶面附近深层砂岩碳酸盐胶结作用和次生溶蚀孔隙形成机理

Mechanism of Carbonate Cementation and Secondary Dissolution Porosity Formation in Deep-Burial Sandstones near the Top Overpressured Surface in Central Part of Junggar Basin

准噶尔盆地腹部深层超压顶面附近(现今埋深4400～6200m,温度105～145℃)砂岩中广泛出现的碳酸盐胶结作用和次生溶蚀孔隙与超压流体活动关系密切.由于超压封闭和释放引起其顶面附近砂岩中的孔隙压力和水化学环境的周期性变化可导致碳酸盐沉淀和易溶矿物溶解过程交替出现.根据腹部地区超压顶面附近深埋砂岩成岩作用、碳酸盐胶结物含量、储集物性、碳酸盐胶结物碳、氧同位素和烃源岩热演化模拟等资料的综合研究表明:含铁碳酸盐胶结物是主要的胶结成分,长石类成分的次生溶蚀孔隙是主要的储集空间类型,纵向上深层砂岩碳酸盐胶结物出现15%～30%含量的地层厚度范围约在邻近超压顶面之下100m至之上大于450m,碳酸盐胶结物出现大于25%含量高值带分布在靠近超压顶面向上的250～300m的地层厚度范围,超压顶面附近砂岩中碳酸盐胶结物含量高值的深度范围也是次生溶蚀孔隙(孔隙度10%～20%)发育带的范围;因晚白垩世以来深部侏罗纪煤系有机质生烃增压作用,导致在超压顶面附近砂岩晚成岩作用阶段深部富含碳酸盐的超压流体频繁活动,所形成的碳酸盐胶结物受到了明显的与深部生烃增压有关的超压热流体和有机脱羧作用的影响;超压流体多次通过超压顶面排放,使得超压顶面附近砂岩遭受了多期酸性流体溶蚀作用过程,形成了次生溶蚀孔隙发育带.

The carbonate cementation and secondary dissolution porosity in the deep-burial sandstones near the top overpressured surface （present-day depths： 4 400-6 200 m and temperatures： 105-145 ℃ ） in the central part of Junggar Basin are intimately related to the overpressured fluid activities. Periodic decreases of pore pressures and changes of formation hydrochemical conditions as sealing and releasing of overpressures along the top overpressured surface can result the processes of carbonate precipitation and aluminosilieate dissolving in the sandstones. Based on the data of diagenesis, carbonate cement content, sandstone property, carbon and oxygen isotope of carbonate cement, thermal maturity modeling of source rocks and so on, integrated studies indicate as fellows： The ferroan carbonate cements are the dominated cements formed in the late diagenesis stage, and the feldspar-related secondary dissolved pores are the most important pore types in the deep-burial sandstone reservoirs. The thickness scopes are more than 550 m, occurring 100 m below the top overpressured surface and over 450 m above the sur face, while the carbonate cement contents reach or exceed the values of 15% to 30%. The thickness intervals are 250-300 m above the top overpressured surface while the carbonate cement contents reach or exceed the values of 25%. The depth inter vals of up to 10%-20% secondary dissolving porosity are concomitant with the depth ranges of relative high carbonate cement contents. Since the late Cretaceous, the carbonate-riched fluid flows have been produced frequently as the development of hydrocarbon generation pressures from the deep Jurassic coal-bearing strata, and the carbonate cements have been influenced evi dently by the overpressured and thermal fluids, implying that the late-stage ferroan calcite carbon is partly originated from the organic-matter thermal degradation. The secondary porosity zones in the deep-burial sandstones near the top overpressured sur face have been formed during the processes of acid-fluid expulsion and migration due to upward releases of overpressures along the diagenetic barrier associated with the top carbonate mineralization zones.