High Resolution Sequence Stratigraphy And Reservoir Characteristics Of Yanchang Formation, Upper Triassic Series In Yangchang Oil Province, East Of Ordos Basin

Ordos Basin, which contains abundant petroleum resource, is the second largest sedimentary basin in China. The Yanchang Oil province that is in the east of the basin is the earliest discovered and developed oilfield in China mainland. The petroleum exploration and development in the area has experienced almost a century. The main oil-bearing layer is Triassic Yanchang Formation. The oil accumulation belongs to lithologic reservoir or structure-lithologic complex reservoir with the characteristics of low permeability or very low permeability.With the development of petroleum exploration and exploitation, the contradiction that different oilfield has different stratum correlation, reservoir classification and evaluation standards is increasingly serious. So it is urgently required a new theory and way to establish a uniform stratum correlation and reservoir classification standard, in order to study the pattern of sedimentary facies and sand-body distribution, evaluate the reservoir and detect the distribution area of favorable reservoir within the chronostratigraphic framework.By the use of a vast amount of drilled cores, well logs and other analytical data in the studied area, combining the observation and description of field outcrops, the author made a high-resolution stratigraphy correlation for the Yanchang Formation, Upper Triassic Series in Yanchang oil province, established the sequence stratigraphic framework of the Yanchang Formation in the first time, studied the distribution features of reservoir sand-bodies within the sequence framework, evaluated the reservoir of the main oil-bearing formation, discussed the influential factors of the reservoir properties. The main conclusions and cognitions can be summarized as follows:1. The paleocurrent direction, lithofacies component and types of sedimentary facies of the Yanchang Formation in Yanchang oil province have been systematically studied based on the observation of outcrops and cores. It is considered that the paleocurrent direction was NE to NNE, there are 8 kinds of lithofacies and 3 kinds of sedimentary facies, i.e. lacustrine facies, delta facies and fluvial facies (including braided river, meandering river and anastomosing river).2. Based on the relationship between cores and log curves, 8 kinds of typical log-facies that can represent depositional features have been identified in the Yanchang Formation, and the log-facies models of the Yanchang Formation in the studied region have been established. Using the models, the author analyzed the sedimentary microfacies distribution of the main oil-bearing formation.3. Integrating the field outcrop, drilled core and well log data, three kinds of sequence interface have been identified, i.e. unconformity surface, flooding surface and accretion / progradation (retrogradation) turnaround surface. Among which there are 2 regional unconformity surfaces(SB₁,SB₅) that are located at the bottom and top of the Yanchang Formation, 3 descending to ascending base-level turnaround surfaces (SB₂,SB₃,SB₄) that are situated at the bottom of Chang-8 Chang-6¹ and Chang2 oil-bearing formation, 3 ascending to descending base-level turnaround surfaces (MFS₁, LFS, MFS₂) that are located at the top of Chang-9 oil-bearing formation, in the middle of Chang-7 and Chang4+5 oil-bearing formation. These sequence interfaces are good isochronous surfaces.4. Based on the basic principles and ways of high-resolution sequence stratigraphy, the author carried out the long term, medium term and short term base-level cycles identification, and divided the Yanchang Formation into 4 long term (LSC₁,LSC₂,LSC₃,LSC₄), 12 medium term (MSC₁ to MSC₁₂) and 31 short term (SSC₁～SSC₃₁) base-level cycles. After correlating each of the base-level cycle, the high-resolution sequence stratigraphic framework of the upper of the Yanchang Formation is defined.5. The petrology features, diagenesis, micro-pore structure, petrophyscial properties of the reservoir of the main oil-bearing formation ( MSC₆ to MSC₁₀ ) are analyzed. The research results indicate that the reservoir sandstone of the Yanchang Formation has the characteristics of low composition maturity and high texture maturity. It's mainly composed of arkose with little feldspathic lithic arenite. The sandstone underwent compaction and pressure solution, cementation, replacement, dissolution and cracking during the diagenesis evolution process. The reservoir mainly belongs to very low to super-low permeability reservoir. The pores in the sandstone mainly include residual intergranular pore, feldspar resolution pore, lithic fragment resolution pore and laumonite resolution pore. The pore structure is mainly consists of big pore - medium to fine throat, medium pore - medium to fine throat, small pore - medium to fine throat and small pore - fine throat.6. On the basis of petrophyscial facies, integrating store capability and production capability, the reservoir of the Yanchang Formation is classified into 5 types, 11 sub-types. The Chang2 ( MSC₆) reservoir has better petrophysical facies. Its reservoir is mainly of II_B sub-type, with little III_B, II_A, I_A. I_B subtypes. Yet Chang6 and Chang4+5 ( MSC₆ to MSC₇ ) reservoirs have poorer petrophyscial faies. Their reservoirs are mainly of III_C and IV_B sub-types, with little IV_C and IV_D sub-types.7. The factors that controlled the reservoir development degree, petrophyscial property and distribution in the Yanchang Formation are discussed. It is considered that the reservoir condition is related with varied factors, including sedimentary environment, diagenesis and tectonic movement, especially with its location in the formation.