Research On The Upper Triassic Reefs And Their Petroleum Geology Conditions In The Western Sichuan Basin

The siliceous sponge reefs in the upper Traissic Maantang Formation in the western Sichuan basin are the largest ones in the world. Meanwhile the Maantang Formation is also the latest marine carbonate layer in the Sichuan basin. So the study on the Maantang Fm., especially on the reefs within the formation, is of great significance to the characteristics of siliceous sponge reefs in the world and the evolution of the Sichuan basin.The dissertation has the investigations on 22 reefs in the upper Triassic Maantang Fm., 10 ones of which are put detailed description, section measure and sampling. Through measured sections, systematic sampling and analysis of the thin sections, it is concluded the carnian reefs is mainly constructed by the hexactinellid sponge and the cyanobacteria in thrombolites, which based on the homoclinal carbonate ramp from 30m to 200m below the sea level and whose primary reef-building process is baffling. The fossils are commonly observed, such as sponge, crinoid, foraminifer, brachiopod, bivalve, bryozoan, worm, in these reefs, in which essentially in place sponge groups are revealed. The subfacies in the reef complex could be easily distinguished. The reef base is ca. 30-60m in thickness and mostly composed of gray bedded oolitic limestone, sometimes relatively thin sparry bioclastic limestone in the top of it. The reef core, the reef flank and the interreef are composed of caesious or black gray bioclastic limestone, and the kind of their fossils is not obviously different but their occurrence state is different each other. The interreef is bedded and their thickness is thinner than the reef core's and the reef flank's, which are block. The rock of the reef flank is looser the one of the reef core after they are weathered enough. The reef top is composed of black bedded mudstone, in which bivalves are common seen and the fauna in which varied from the one in the reef core & flank. The silty mudstone under the Carnian third-sequence boundary induced the extinction of the reefs in the Maantang Fm.The reefs have a relative topographic relief comparing the contemporaneous sea floor, and the height of every reef is from 2m to 70m. The outcropping area of the reefs is along the Longmenshan Belt and the length of the area is ca. 65.6km in northeast direction, and is 1-2km in width.The growing, developing and extinction of the reefs in the Maantang Fm. may indicate the evolution process of the Sichuan basin and the adjacent area. The reef base (i.e., the oolitic bank) develops in shallow water and high energy without terrigenous material. Then the increasing terrigenous material is deposited and the depth of the water became deeper. The surroundings around reefs turned into be feculent and be closed so that the reefs died away. Consequently the silty mudstone in the Xiaotangzi Fm. is deposited. The process could coincide with the movement of the orogenic belt on the edge of the Sichuan basin. The sediment is not exclusively carbonate during Carnian in the western Sichuan basin. The northwestern Sichuan basin has more and more terrigenous material up to the top of the Maantang Fm. That suggested the mountain of the northern Sichuan basin has uplifted and become the provenance. The upper part of the Maantang Fm. is also increasingly influenced by terrigenous material. And the Maantang Fm. in the Longshen Well 1 includes the lower part (the thin layer oolitic bank) and the upper part (a large suite of mudstone). The overlying formation of all the sections of the Maantang Fm. is the Xiaotangzi Fm., main including silty mudstone. The sedimentary process should be related to the gradual raise of the Qinlin old land and the Longmenshan orogen belt.The Bank/Reef of the Maantang Formation underwent shallow sea, deep sea, early diagenesis, middle diagenesis, late diagenesis, epidagenetic stage. There is not obviously early exposed in the Maantang Fm. The primary pores of the Bank & reef have been almost filled and the secondary pores do not develop well. So the porosity & permeability of the Maantang Fm. is considerably low. For the analysis of the samples from the outcrops, the Bank/Reef only reaches the level of non-reservoir or low yield-reservoir. However it is similar between the siliceous reefs in the western Sicuan basin and the Oxfordian siliceous reefs in Europe. Comparing with the serials of the Oxfordian reef, the shallow-sea, high-energy reef may be discovered near the central Sichuan Basin in that depression. They should be a target for oil/gas exploration.