SEDIMENTARY FACES DURING HUANGLONG STAGE OF MID-CARBONIFEROUS IN EASTERN SICHUAN AND ITS CORRELATION WITH OIL AND GAS

The Huanglong formation of Mid-Carboniferous in eastern Sichuan is mainly tidal flat deposits, in which have been discovered six gas fields and six gas-bearing .regions since the drilling out of high-productivity natural gas in 1977. Through the exploration practice in recent years,it has been confirmed that, besides the conditions of trapping and preserving, the key in exploration is closely related to the stretch of sedimentary facies zones. The eastern Sichuan was an estuary, Land-bounded on three sides during Mid-Carboniferous age, the transgression area successively enlarged westward from the east,and the tidal channel was situated nearby Zimutang of Jiansi county in western Hubei. Durng Leiyinpu time (C₁²), the realm of sedimentation was relatively smaller,with sediments of tide-above zone as the main, lithology of which is breccia- limestone, limestone intercalated with dolomites and gypsum rock) during Fuchenzhai time (C₂²), transgression expanded westwards, with sedimentary facies changed into intertidal zones as the main, lithology of which is mainly breccia-dolomites and dolomites, intercalated with bioclastic breccia-dolomites and bioclastic dolomites of relative abundancejand during the Chuantong (eastern Sichuan) time (C₂³), transgression broadened continuously, though it was still mainly intertidal zone,yet the tidal channel was not so obvious,the sea-water advanced and retreated fluently, lithology of which had changed into limestone, breccia-limestone and bioclastic limestone as the main, intercalated with dolomites, breccia-dolomites and bioclastic dolomites. The maximum residual thickness is 91m, in which always can be seen such sedimentary characteristics as sun-cracks, laminations, gravels in gravel, bioturbate structure stromatolites, birds-eyes, biogenic burrows, micrite envelope, oncolite, tents, cross-beddings,scouring surfaces, geopetal fabrics, salt-casts ts, gypsum traces, and so on. Organisms are comparatively abundant but broken, always mixed with those of different water realms, such as algae, foraminifera, fusulinids, echinoderms and brachiopods. As the tidal flat sediments are the main,the Carboniferous Huanglong formation in eastern Sichuan has neither oil-generating environment, nor sedimentary facies with ideal conditions of oil accumulation and preservation. Owing to the confinement of the sea basin during sedimentation and the dryness of climate,the calcium carbonate lime which had deposited on tide-above and intertidal zone has beene generally replaced by dolomite so as to form parasyngenetic dolomite; although this kind of dolomite was replaced mainly in volume, and having micro-and silt-crystalline texture,yet their rhombic crystals are fair and the crystalline pores are developed and their mutual communication may be seen under electron canning microscope. And what is still more important is that the intertidal sediments, exposed to the water surface, through successive contraction by dryness, had abundant gravels and grains, and often formed rather developed pores between grains (gravels)and microfractures, thus providing necessary conditions for fresh water to corrode and expand porosities hereafter,and leading to commonly rather better porosities of those which had the developed parasyngenetic dolomites in the Mid-Carboniferous Huanglong formation in eastern Sichuan for example, the intertidal flats both higher and lower etc. during Fuchenzhai time, are such the case, and the gas fields are almost present in the parasyngenetic dolomites of these inter tidal zones. For this reason, the reservoir condition of Carboniferous Huanglong formation in eastern Sichuan, generally speaking, is rather good, but the secondary solution porosities related to sedimentary facies predominate and not the primary porosities formed during sedimentation. In this paper, some problems have been dealt with, such as the especially wide distribution of tidal flat sediments of Carboniferous Huanglong age which surpasses by far the breadth of each tidal region along the sea in the.