The Study On Structure Feature Of Longwangmiao Formation In Gaoshiti-moxi Area,Sichuan Basin

On the basis of previous studies, a large number of seismic data and well logging data were used to explain the holistic and tiny structure feature of seismic profiles in Gaoshiti-Moxi area, aiming to analyze and discuss the structure feature of Longwangmiao formation elaborately. Using Layer Flattening Technique was to detail the feature of each structural period in tectonic evolution stages of the study area, and to analyze the effect of these series of tectonic activities on Longwangmiao formation reservoir forming finally.The surface topography nowadays is relatively single, the terrain slopes gently, and no obvious tectonic activities: folds uplift weakly and no large-scale faults develop. Deep structure overall performed as a north-east uplift like a nose on the ancient palaeohigh, characterized by northeast pitching, multi-rows and multi-high points, existing two axes: the south – north one(Gaoshiti area) and the north-east one(Moxi area). Some topo-tiny-faults are developed in Gaoshiti structure trap and in Moxi structure trap topo-tiny-faults and high points are developed too. Longwangmiao formation is relatively flat today, with roughly the same thickness, and its topo-high point locates in study area. The formation develops low angle gently folds and small secondary folds, topo-faults separate large-scope and no large-scale fault. In the whole Sichuan Basin, the structure top point of Longwangmiao formation locates in Ziyang-Weiyuan area, but the study area is still at the topo- high position.Before Longwangmiao formation sedimentated, the study area had experienced a series of tectonic activities and the ?fill up? effect of formations, till the Longwangmiao formation sedimentated, the surface in study area was almost flat. Longwangmiao formation and Gaotai formation were sedimentated steadily with almost the same thickness. After Longwangmiao formation sedimentated, the study area roughly experienced five evolution periods:(1) Early Cambrian- Early Ordovician: steady sedimentation period. In this period, along with the increase of Middle Cambrian transgression, formations of Middle-Late Cambrian sedimentated steadily.(2)Early Ordovician- Early Permian: Caledonian palaeohigh period. During this period Caledonian movement made the whole Sichuan Basin lifting, forming the macroscopic tectonic framework of palaeohigh and caused the losing of large numbers of strata. Later Hercynian movement made the palaeohigh grow further, causing more loss of strata. The top point of Longwangmiao formation located in West-Basin this time(denudation to Dengying Formation);(3)Early Permian-Middle Triassic: overall lift and down period. In Early Indosinian, structure movements in study area were relatively stable, the sedimentation of strata made the whole area subsided and tectonic deformation was weak. To the end of Middle Triassic, Indosinian movement strengthened, made Kaijiang and Luzhou rose and began to form the palaeohighs, The study area was also in the uplift area, strata denudated and tectonic position began to be higher than the west and north Sichuan Basin. The top point of Longwangmiao formation migrated from west to east gradually and ancient oil reservoir began to form in this period;(4) Late Triassic- Early Jurassic: structure reverse period. In this period, the tectonic activities of periphery Sichuan Basin became stronger, the basin began to move into continental basin evolution stage, Longmenshan in the northwest began to upheaval, Western Sichuan Depression began to form, the top point of Longwangmiao formation continued to migrate to the South-west direction, the study area became to be structural high points gradually because of its stability, the structure of Longwangmiao formation in study area reversed to high position from low position. Ancient oil reservoir also started to cracking to ancient gas reservoir during this period, migration and accumulation to structural high part(that is study area);(5) Middle Jurassic-Now: steady lift and down period. In Yanshanian, the whole Sichuan Basin moved into continental basin evolution stage completely. Western Sichuan Depression continune developed, the study area became structural high point completely; During the Himalayan period, Weiyuan anticline in the southwest part besides study-area uplift rapidly, the structural high points migrated to Weiyuan area, and the study area became topo-high point in the north east wing of the anticline. Overall, although the structural change was very huge around the study area, the area itself was whole-slab lifting, no big tilted effect happened, trap persist. The Longwangmiao ancient oil reservoir got some topo-adjustment during these series of transformation, and finally setting for today's gas reservoir. The ancient tectonic axis of Longwangmiao formation gradually migrated to the southwest in the evolution process. The structural high point of the basin migrated from Ziyang- Suining to Ziyang – Weiyuan. The Longwangmiao oil(gas) reservoir completed its changing from ancient oil reservoir to ancient gas reservoir to gas reservoir today. The key period began from Middle-Late Indosinian, traps were formed in the study area, which was beneficial to the old oil and gas transported together. At the same time, the particularity of tectonic position, the bottom of the superior conditions of hydrocarbon source rock, the high quality reservoir spaces and the high quality cap rock conditions in the study area provide conditions of correlation, today's gas reservoir are eventually formed.