Research On Sequence Stratigraphy Of The Upper Permianseries Changxing Formation And The Lower Triassic Fei Xianguan Formation In Western Sichuan Basin

The western Sichuan Basin is an important oil and gas exploration basin and the northwestern as well as Sichuan adjacent areas have achieved the breakthrough for marine gas exploration. Western Sichuan has the similar tectonic and sedimentary evolution history with the adjacent areas and its marine gas accumulation conditions are superior, especially the marine strata which is close to hydrocarbon generation depression to the edge of platform has enormous of marine oil and gas resources in western Sichuan basin. In this paper,through detailed analysis of the core, drilling and logging data, the sequence stratigraphy research on the upper Permianseries Changxing Formation and the Lower Triassic Fei xianguan Formation in the western Sichuan Basin carried out. According to the establishment of sequence stratigraphic framework and isochronous trellis sedimentary system, their sedimentary facies and subfacies types are identified. On the basis of the sequence stratigraphic framework research combined with analysis of individual1well and associated well data, lithofacies-paleogeographic can be draw up with level3sequence as mapping unit for marine strata sequence segment.and key reservoir layers of the upper Triassic-lower Triassic in the study area.The planar distribution and space-time evolution laws are revealed to prepare well for reservoir prediction and indicate the direction formarine reservoir evaluation.The location and evolution characteristics of the ancient uplift within the platform and western boundary passive continental margin of the upper Yangtze plate are confirmed thourgh the lithofacies-paleogeographic map. Ultimately, by studying different structure types of platform and roles that each carbonate microfacies controlled reservoir development and reservoir properties within different facies belt, favorable carbonate reservoir facies belts of different periods.can be forecasted.The specific content of the paper are as follows:1.Through the main profile observation to summarize systematically sequence boundary genetic types of marine Permian-Triassic, two kinds of sequence boundary genetic types are identified:local exposure erosional unconformity and lithologic facies conversion interface in which the local exposure erosional unconformity interface is related to reservoir reconstruction including two sections:between the second section and the third one of Feixianguan formation as well as the Feixianguang and Jialingjiang formation. According to the classical theory of sequence stratigraphy and the basis of the sequence key interface identification, two level3sequences are divided of the Upper Permian Changxing Formation:SQ1, SQ2that both are sequence Ⅱ,in which transgressive systems tract (TST) and highstand systems tracts (HST) developed; also two level3sequences are divided of Lower Triassic Feixianguan Formation:SQ3, SQ4, in which transgressive systems tracts (TST) and highstand systems tracts (HST) also developed. On the basis of the above, sequence stratigraphic contrast research carried out between drilling wells to expound the water depth, accommodation space and base-level change during each sequence depositional period of different directions and then ancient geographic and paleogeomorphology change characteristics during each sequence development period would be discussed further and the possible reservoir development zones can be pointed out. Generally, that the west and north are deeper than the south and east indicated that the base level rise and accommodation space growth rate of the north and west is larger so that the north and west are low but the east and north are high in ancient landforms.2. According to the sedimentary facies combination characteristics, three kinds of depositional system can be identified:shoreland sedimentary system, edge platform system and continental shelf-basin system. Then six kinds of sedimentary facies and11types of sedimentary subfacies are divided into, namely, shallow sea shelf facies, basin, platform margin slope, platform margin reefs, open platform facies and tidal-flat facies. Based on sedimentary facies characteristics, sedimentary facies model are summarized that is edged platform depositional model in the study area.3.After sequence stratigraphic framework establishment, this paper systematically finished reservoir section lithofacies-paleogeographic map compilation with the system tract (equivalent to level4sequence) as a unit for the first time. Research shows that level3sequence transgression system tract often presented certain succession with high systems tract within the study region that is from TST to HST; the phase sequence changed from shallow to deep to shallow; plane phase belt exhibition presented from TST to HST; deep-water sedimentary area gradually narrowed while shallow-water depositional area gradually expanded. Through analysis for the limestone, dolomite, particulate carbonate rocks isopach map of each sequence frame, reservoir exhibition characteristics are clear.Finally, according to each sequence lithofacies palaeogeographic characteristics combined with tectonic pattern of each top formation ancient and modem structural map in the study area, the ancient geomorphic evolution characteristics of the upper Permian-lower Triassic can be expouned.4.By studying the depositional systems spatial distribution, assembly characteristics and ancient landscape development situation in the study area, carbonate platform structure types can be summarized. It pointed out edged carbonate platform existence of upper Permian-lower Triassic in the study area then expounded its structure characteristics and its facies ntrol and reservoir control roles. Studies have shown that the edged carbonate platform controlled development of platform, platform margin reef facies and platform margin slope facies while the platform margin reef flat fades controlled reservoir development and it is sea level changes cycles that controlled the platform margin reef reservoirs distribution and migration during the Changxing-Feixianguan depositional peirod. Then it analyzed the functions that sequence stratigraphic controlled the reservoir.Combined with the reservoir physical property data characteristics,it comprehensively researched and forecasted the reservoirs in the study area. The platform edged reef-bank reservoirs of Changxing-Feixianguan Formation are considered as the main favorable layers in the study area.