| Chengdu sag is the plain between southern Longmen mountains and Longquan mountains, Zonggang mountains in the west part of Sichuan basin, with the sites about 5000km. There is a xinchang tectonic belt uplift to NEE in the north of the sag, and a dayi-anxian tectonic belt distribution for NE-SW in the west of the sag, and zhixin tectonic belt distribution for SN in the east of the sag. The research area belongs to Chengdu, Chongzhou, Pengzhou, Dujiangyan, Deyang, Guanghan and Shifang in administrative regions. And the research area is a weak tectonic deformation area, the present study and exploration of Shaximiao formation are pengxian-deyang syncline area in northeast, xiao-xin-he tectonic belt area in north, and xindu-chengdu gently sloping tectonic belt area in south.Chengdu sag has experienced indosinian, yanshan and Himalayan tectonic movement, and in the late Triassic, Wenjiang and Majing areas began to form structure, while Xindu-luodai area has not formed structure; in the early Jurassic, Wenjiang> Majing, Xindu-luodai area began to form structure gradually, at the same time, the size of tangent in Deyang-guanghan area has increased while formed the positive structure; in the late Jurassic, with the inheritance of uplift in Wenjiang texture, the closed area and amplitude are decreased gradually; until the end of the early Cretaceous, shaximiao formation has begun to uplift continuely, with the extrusion tectonic movement and the development of the fracture of the longquan mountain, anticline has begun to be formed in Xindu-luodai regions:under the influence of Himalayan movement the fracture develops in the south-east of Majing anticline, and the activity runs through the whole period of the development of Majing anticline; in the late Himalayan movement, the anticline in Xindu-luodai region formed the anticline structure in Luodai and the nose-like structure in Xindu today under the extrusion of tectonic movement.The lower shaximiao and the lower qianfoya formation show the "guankou sandstone" of unconformity interface at the bottom of the gradual overlap unconformity contact on the profile; and the "leaf segments interface shale" between the upper and lower shaximiao formation is a symbol of the interface identification, while not all of the symbol is the unconformity interface; suining formation and the upper shaximiao formation is characterized by the related integration surface inside the basin, and shows phase transformation interface by coarse to fine in vertical. The shaximiao formation could be divided into three segments:the lower shaximiao formation, the down side of upper shaximiao formation and the up side of upper shaximiao formation. And the shaximiao formation could be divided into 11 sand formation further, including J2s33 J2s23、J2S13、J2S42、J2S32、J2S22、J2S12、J2S41、J2S31、J2S21、J2S11.By the analysis of lithic sandstone type distribution, the characteristics of heavy mineral combination (ZTR index) and the light detrital component, combining the geomorphology feature around Chengdu sag, there are two main source directions in Chengdu sag:one is from MiCang mountain-dabashan mountain in the north rim of the basin; the second one is from longmen mountain in the north-west rim of the basin. Chongzhou-pixian-pengzhou regions mainly receive provenance from the Longmen mountain in west; Deyang-jintang-luodai regions mainly receive provenance from the Micang mountain-daba mountain in north-east; Majing region and xinfan region show a complex provenance in central area, with the influence of far source in north-east and north and the near source in Longmen mountain in north-west.With the researches of the sedimentary facies of the Jurassic shaximiao formation in Chengdu sag by predecessors, in this article, through the core observation and description, and the analysis of test data and logging data, the Jurassic shaximiao formation in Chengdu sag has two main sedimentary system in east and west:one is a short axis of the braided river delta sedimentary system in western and the other one is a long axis of the meandering river delta sedimentary system in eastern. The provenance in western sag is mainly from the middle part of longmen mountain, and takes the shape of the short axis of braided river delta aiming to northwest-southeast, with the schistose distribution of sand body; the provenance in the middle and east part of Chengdu sag is form the north part of longmen mountain and micang mountain, and takes the shape of long axis of meandering river delta aiming to northeast-southwest, with the zonal distribution of sand body. The development of subfacies show that there are delta plain, delta front and front delta subfacies, and the sedimentary microfacies include distributary channel, underwater distributary channel, distributary bay, mouth bar, far sand dam.The main difference between short axis of the braided river delta and long axis of the meandering river delta is the average particle size of sediment, the sedimentary granularity of the braided river delta is much larger than the granularity of the meandering river delta; the other difference is the braided river delta sedimentary system develops no or less far bar microfacies. This paper selects several single well with different resources contrasting the differences between the two delta depositional systems.There are eight major sand formations including J2s33 and J2S31 in upper shaximiao formation, all sand formations in the down side of upper shaximiao formation, J2S14 and J2S11 in the up side of upper shaximiao formation. The sand body distribution characteristics of sand formations are different from each other, this article analysis the main characteristics of sand body distribution of different sand formation, preparing for the next step in the exploration and development.The procession of the Shaximiao formation formed development shows the rising process of lake plane, although the internal also appears many small lake level up and down repeatedly. In the lower shaximiao formation sedimentary period, the sand body in Chengdu sag has a big development, and the thickness of sand body is much larger, mainly the delta front deposition; in the down side of upper shaximiao formation sedimentary period, Majing-shifang and Chongzhou-pian regions sediment delta front and delta plain depression, Guanghan-jintang and Xindu-luodai regions sediment delta front subfacies in Chengdu sag; in the up side of upper shaximiao formation sedimentary periods show the continuation section of sedimentary framework of the down side of upper shaximiao formation, with the delta front area reducing.The dynamic evolution of the sedimentary process shows different sedimentary system in different periods of distribution range and it is inheritance, but there exists certain differences. The rise and fall process of lake water change more frequently in sedimentary period of shaximiao formation from the section, and the development of sand body in shaximiao formation is different on the longitudinal, mainly depending on the basic level cycle. The sand body d evelops mainly in the early rising datum in the middle of the development, then in the later rising datum. The sand body mutually superpose in different period, making the overall thickness of sand body much bigger in vertical in the study area, and showing a better continuity in the plane distribution. The sand body is the favorable part for oil and gas gathering and storage. |
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