| Recently, oil and gas exploration in shallow reservoirs in north Sichuan Basin has been characterized by multilayer, low porosity and permeability, hypertension, local enrichment and high productivity. The Xujiahe Formation was the typical representative. Currently, many drilling wells have got obvious gas production tests in Xujiahe Formation, Northeast Sichuan. The exploration effect in east Yuanba-Renhechang area took the most prominent position, and only in well L28, the forth Xujiahe Formation received 102.36 million cubic meters of natural gas per day. Tongnanba area, west Yuanba area also obtained industrial gas flows and achieved great breakthroughs in conglomerate section in the forth member of Xujiahe Formation. The results have testified that the forth member of Xujiahe Formation has good exploration potential in clastic strata, north Sichuan Basin. However, the exploration and research work in the north Sichuan Basin have been mainly focused on the second and third member of Xuajihe Formation, the basic geological research work in the forth member of Xuajihe Formation were not enough and systematic.On the basis of previous research work, through rock thin section, geochemical analyses, logging data, field survey, core observation and other means, the direction, properties and distribution of provenance, the type and evolution of sedimentary facies, the characteristics of depositional system have been positively determined.Combined the objective layer tracking, the stratigraphic comparison and the continuous variation principle of thickness, with the help of eight profiles of stratigraphic framework in both vertical and horizontal direction in study area, the strata division and contrast of the forth member of Xujiahe Formation has been done. There has been great thickness variation in the forth member of Xujiahe Formation. From west to east, the overall thickness decreased at first and then increased, while from north to south, the thickness tended to be stable and decreased slightly. The thickness center of the forth member of Xujiahe Formation was located in east Yuanba area.Based on field outcrop comparation, clastic component analyses and heavy mineral assemblage, it turned out that Longmen and Daba mountains provided source for the forth member of Xujiahe Formation in north Sichuan Basin.But through the research work in well L17 and well L4 in eastern Yuanba, we found that the upper part of the forth member of Xujiahe Formation mainly developed feldspar lithic sandstone. Combined with the imaging logging results in well L17, eastern Yuanba was influenced by NNE oriented provenance in the sedimentary period of the forth member of Xujiahe Formation. Overall speaking, the forth member of Xujiahe Formation in North Sichuan Basin was controlled by three source systems, respectively, NW, NNE and NE.On the reference of paleocontinent rock characteristics in peripheral orogenic zones, the NW source rocks were mainly the early-middle Triassic marine carbonate strata. The NNE source rocks were Nanhua to Silurian from the Qinling orogenic belt, containing volcano rocks, siliceous rocks. In the late Triassic, NE provenance had an increasing impact on study area, affecting a wide range, developing large sedimentary thickness, seeing a lot of quartzose, cherty gravel in depositional area. Combined with regional geological data, the NE source rocks were mainly from Devonian in south Qinling belt.Previous studies showed that different provenance systems have different mineral composition. The NW provenance system mainly affected the western Yuanba area, developing siliceous and calcareous conglomerate and glutenite deposits and then gradually turnning to calcarenaceous sandstone in the basin. The NNE provenance in basin edge developed conglomerate, containing low-grade metamorphic rock and volcano rock, and then turnning to lithic sandstone and lithic quartz sandstone in the basin. The NNE provenance system provided the main source for study area, extending a wide range, mainly affecting Puguang to Bazhong area. Central Yuanba was the meeting zone of three different oriented provenance systems.Through the comprehensive analysis of the rock type and structure of the forth member of Xujiahe Formation in study area, combined with the characteristics of logging response, the results showed that the research area has various types of facies marker. The rock types included conglomerate, glutenite, coarse sandstone, fine sandstone, siltstone, mudstone and shale. The cumulative probability curves of grain size in study area were mainly divided into three sections. That could be further divided into three types. The sedimentary structure developed in study area include scour surface, massive bedding, parallel bedding, cross bedding, horizontal bedding, wavy bedding, deformed bedding, which indicating obvious changes in sedimentary hydro-power. Shapes of logging curve were divided into the box and the gear box, bell, funnel, finger. Combined with logging data, the logging-sedimentary response model of the forth member of Xujiahe Formation in study area was established.Comprehensive analyses of sand body distribution and seismic attributes characteristics of the forth member of Xujiahe Formation in study area, it found that during the deposition, the sediments were under shallow water, controlled by different direction of provenance, developing braided river delta sedimentary. The braided river delta sedimentary included braided delta plain sub facies and braided delta front sub facies. The important basis for distinguishing braided river delta plain and front was the development of large scale of conglomerate and sand conglomerate. Based on sedimentary facies analysis, characteristics of sedimentary system distribution had been summarized that sedimentary system under different provenance system had different characteristics, resulting in big differences in different sediments, including the distribution of the sedimentary system, the range of micro facies, the thickness of sand body, the composition of rocks, etc. |
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