Study On High Resolution Sequence Stratigraphy And Sedimentary Facies Of The Upper Jurassic Penglaizhen Formation In Western Sichuan Depression

The Western Sichuan Depression Penglaizhen Formation Gas Reservoir is currently the main gas in southwest oil and gas branch company of Sinopec with higher potential of exploration, abundant remaining resources. However, the degree of prospecting is inhomogeneous, and natural resource is in the critical period. Based on the theory of high resolution sequence stratigraphy and sedimentology, by the method of multidisciplinary analysis such as application of geological, logging, and seismic, in this paper the high resolution sequence stratigraphy and sedimentary facies of Sichuan depression Penglaizhen formation are researched.Based on previous research achievement comprehensively in this paper, by means of field outcrop, drilling core, logging and seismic data to identify sequence boundary, western Sichuan depression Penglaizhen formation will be divided into two kinds of sequence boundaries, the unconformity surface and erosion, and flooding surface, thus, to expound the characteristics and identification marks of different-level sequence interface. The sequence structure, stacking pattern has been analysed to determine the classification scheme of the western Sichuan depression Penglaizhen sequence which was segmented into2third-order sequences (long-term cycles),4grade sequences (middle-term cycles) and29V-grade sequence (short-term cycles). It is found that the mid-term cyclic sequence has good comparison relationship with stratigraphic units, and short-term cyclic sequence has fine parallelism with sand unit so that it will be convenient to meet production needs.According to the developmental situation about ascending half cycle and the descending half cycle of datum plane cycles base-level cycle sequence, study on base-level cycle sequence structure characteristics was divided into3types of sequence structure, namely the upward "deeper" asymmetric cycles (A), asymmetric cycles to "shallow"(B) and symmetry turning upward "deeper" then "shallow"(C type) which can be further divided into3subtypes, so that further study on the characteristics of long-term, medium-term, short-term cyclic sequence stacking pattern, profile structure and sedimentation model. Research shows that the short-term cycle structure presents diversity characteristics, various types of cycle structure were developed, among of which the type C sequence structure is as its main development type, then type A, and type B develops relatively less. Medium-term cycle is given priority to type C1and C2cycle structure. Long-term cyclic sequence is well preserved and only the Sedimentary thickness of developed cyclic sequence at base-level ascending half cycle and the descending half cycle is approximately equal to C2cycle configuration. Above different deposit system or same deposit system in different position have different characteristics of the sedimentary sequence and evolution.On the basis of comparison principle and method about high resolution sequence, and division about281wells in different base-level cycle,15wells which consisting of 82well drilling throughout the study area have been compared in whole area in this paper. The line of demarcation (sequence boundary) between long-term and medium-term cycle sequence is preferred as the isochronous comparison area, the short-term cycle sequence is as the isochronous stratigraphic unit, in order to establish the high accuracy isochronous stratigraphic framework in the Western Sichuan depression Penglaizhen formation.As a result of the analysis of source, adopting the chemical analysis data of field outcrop and specimens to statistic the conglomerate composition, using the analysis of field paleocurrent and ZTR heavy index, it summarized that in the Penglaizhen formation sedimentary period the source supplied from two provenance area with various direction feature, moreover, the western basin is the most important source region in Western Sichuan depression, while the East is the secondary source area.In the provenance analysis basis, by the control of the unified stratigraphic framework in the whole area, the depositional system planar distribution and evolution characteristics in Western Sichuan depression Penglaizhen formation have been researched. Research by the method from points progressively, that is, through the observation of10wild field sections and38core analysis, combination with logging data, conclusion the sedimentary facies mark such as rock color, type, structure, sedimentary structure and electric logging curves, the deposit system during the deposition in western Sichuan depression Penglaizhen has been distinguished into alluvial deposit system, delta deposit system and lacustrine deposit system. And the characteristics of sedimentary facies, subfacies and microfacies in each deposit system have been described in details.In this paper the mid-term cyclic sequence selected as isochronous stratigraphic was researched to the sedimentary facies distribution and evolution. To obtain the sedimentary facies distribution characteristics, sedimentary facies plane map has draw, which gained by the corresponding sand ratio of datum plane cycle division and lithologic statistics. Furthermore, due to the change about various types of sedimentary facies and rule changes about plane distribution relationship, it summed that there was a sedimentation model during the sedimentary period in western Sichuan Penglaizhen. the one was the type of Alluvial Fan-River-Delta-River. Also, this paper summarized the evolution process of study area in Penglaizhen formation sedimentary period.