Regulation Of Sedimentary Distribution And Reservoir Characteristics Of The Upper Triassic Xujiahe Formation In Sichuan Basin, China

The Sichuan basin holds multi-cycle features for its complicated course of evolution. The complicated sedimentary framework, pattern of the basin and the hydrocarbon pooling conditions of Sichuan Basin today was created by the long geological time of evolution. As the geological data accumulation and research continues, we continue to discover that the structure sequence is divided too rough, the research definition of sedimentary facies is low, the diagenesis study is lagging, the formation mechanism of secondary porosity is unclear, and so on. The restrictive functions of the lacking in earlier work to the oil and gas exploration is increasingly prominent, which has limited the Predicting the distribution of high quality reservoirs and the delineation of exploration target areas. Moreover the Sichuan Basin experienced multi structural movements in the long geological history. The Indosinian movement in Mesozoic made the sedimentary framework in Sichuan Basin turned from marine to terrestrial conditions, which have a great influence on basin structure, the filled characteristics of deposits and the reservoir forming conditions. So take a further research in the regulation of sedimentary distribution, the controlling factors to reservoir development is essential.This paper use the theories and technical methods in sedimentology, sequence stratigraphy, structural geology, seismic stratigraphy, sedimentary basin analysis, systematically analyzes the sedimentary environments and reservoir features of Xujiahe formation, then reveal the main controlling factors on regulation of sedimentary distribution and reservoir characteristics of the Xujiahe Formation in Sichuan Basin. The main contents results are as follows:(1)On the base of making geological-section in field and rock slices identification interior, and predecessor’s work, we have identified the stratum division and sequence boundary of Xujiahe Formation. The Xujiahe Formation is divided into three structure sequences and six third grade sequences. And the framework of sequence strata in Sichuan Basin is set up.(2)Through profile measurements, core observation and core observation, the Xujiahe Formation of the upper Triassic in Sichuan Basin can be divided into six depositional systems. It contains the alluvial fan sedimentary system, river sedimentary system, delta sedimentary system, lake sedimentary system, coast system。It is believed that the 1st to 3rd members of Xujiahe Formation were deposits of marine-transitional environment, while the 4th to 6th members were terrestrial environment. Using the third grade sequences for mapping units, the structure -lithofacies paleogeographic map of upper Triassic in Sichuan Basin is developed systematically. And the deposits distribution characteristics and its evolvement rules are analyzed, sedimentary models for each evolution stages are set up then.(3)Based on the abrupt and gentle change of basin slope and the fluvial process forms, the delta sedimentary facies of Xujiahe Formation can be divided into two forms of gentle sloped long-axis delta and steep sloped short-axis delta. Take the 4th member of Xujiahe Formation for example, the differences of the two forms are analyzed and discussed. It find out that, the gentle sloped long-axis delta usually shows bird’s foot delta in the plane. It has a large scale, and tis delta lobes often individual, the sandstones in it often have high textural maturity and composition maturity, and fine-grained, mainly in secondary porosity; the size of the delta lobes of the steep sloped short-axis delta are relatively small, and usually connect with each other into banded in horizontal, the sandstone of it often have lower textural maturity and composition maturity than which of the long-axis delta, and coarse-grained, pores poorly developed and with a relatively poor reservoir property.(4)Combine the reservoir property of each drilling, through flake researching, it suggest that reservoir rock types of Xujiahe Formation in Sichuan Basin mainly include the five kinds of feldspathic litharenite, lithic arkose, feldspar quartz sandstone, lithic quartz sandstone and lithic sandstone; the pore types mainly include intragranular dissolve pore, intergranular dissolved pore, intergranular pore. And the diagenesis and diagenetic facies are analyzed.(5)Based on the macroscopic structural characteristics of Sichuan Basin, the Basin in upper Triassic can be divided into four belts include the foreland thrust belt, the foreland depression belt, the foreland slope belt and the foreland uplift belt, and each belt with different reservoir characteristic. Overall, the foreland thrust belt and the foreland depression belt were in bed diagenetic environments, while the foreland slope belt and the foreland uplift belt were in advantage diagenetic environments, it makes the result that the reservoir characteristic of the foreland thrust belt and the foreland depression belt best in all, and the reservoir characteristic of the foreland thrust belt and the foreland depression belt bed. Based on different structure, sedimentary, diagenetic environments, the reservoir types of various structural zones have different standard. Considering the influence of cracks to the reservoir, the under-standard to reservoir of the foreland thrust belt and the foreland depression belt is rather low, while reservoir of the foreland slope belt and the foreland uplift belt become to be pore type reservoir gradually, and the under-standard of which gradually raise.