Research On Depositional Systems And Pool-forming Assemblages Of Chang 1 And Chang 2 Members Of The Yanchang Formation From Ansai-Zichang Area In The Ordos Basin

The relatively small-scale and hidden oil pool of Chang 2 Member of the Yangchang Formation is gradually becoming one of the major aimed prospective members for hydrocarbon exploration and development, and for enhancing oil production, with development of hydrocarbon exploration in the Ordos Basin. The Chang 2 oil pool is widely distributed in northern part of Huluhe and southern part of Qilinggou, where oil exploration difficulties and risks are increasing due to complicated controlling parameters and more concealed intensity of the Chang 2 Member. Thus, research on spatial distribution of the oil pool becomes a key problem for hydrocarbon exploration in the area.Recently, recognition for concealed oil pools are mainly by means of sequence classification and isochroous stratigraphic framework establishment, and spatial distribution investigation of sandstone bodies, using theories and technological methods of sequence stratigraphy. Comprehensive research combining sequence stratigraphy with petroleum system and/or hydrocarbon pool-forming dynamic system is becoming a trend for sequence stratigraphic research, by which we can study the whole processes from source rocks to trap forming, in order to probe accumulation, emplacement and distribution of hydrocarbons in the sequence stratigraphic framework.Sequence stratigraphy is being improved from seismic stratigraphy in basin based-scale to high-resolution sequence stratigraphy in reservoir based-scale in recent years. Enhancement of precision for time-resolution and sequence classification makes it possible that we can establish high-precision sequence stratigraphic framework. Thus, high-resolution sequence stratigraphy is showing more and more advantages theoretically and methodologically. Hydrocarbon exploration deployment needs more and more concrete hydrocarbon-bearing prospect aims due to continuous enhancement of oil and gas exploration level. Thus, research on assemblages of pool-forming has gradually attracted attention and increased interest in petroleum geologists. An assemblage of pool-forming can be defined as reservoirs and traps confined geographically and stratigraphycally in which lithology and depositional facies are related with tectonic characteristics.Precise small-scale classification and correlation of sequence stratigraphy for the Chang 1 and Chang 2 of the Yangchang Formation are conducted, characteristics and evolution of depositional systems, features and distribution of source rocks, reservoirs and seals in the sequence stratigraphic framework are studied in this paper. Meanwhile, types and characteristics of hydrocarbon pool-forming in the sequence stratigraphic frameworkare also elucidated, by using theories and methods of high-resolution sequence stratigraphyand hydrocarbon pool-forming assemblage.Research results show that the Yanchang Formation of the Upper Triassic was formed under a Long-long-term Sea Level Cycle (LLSC1 )and controlled by tectonic movement from 231 Ma to 203Ma, which can be divided into four Long-term Sea Level Cycles (LSC1, LSC2, LSC3 ^ LSC4) included in these Long-term Sea Level Cycles are nine Middle-term Sea LevelCycles (MSCK MSC2......MSC9) from the bottom to the top. The MSC8 (Chang 2 Member)and MSC9 (Chang f Member) consist of seven Short-term Sea Level Cycles (SSC1,SSC2......SSC7). The MSC8 characterized by fluvial system which can be divided into threesub-facies of braided stream (SSC1, Chang 23), meandering stream (SSC2, Chang 2i) and anastomosing stream (SSC3, Chang 2\). The MSC9 is lacustrine-deltaic sedimentary facies, and delta plain (SSC4, Chang U), delta front and shallow lacustrine (SSC5, Chang I3), delta front and delta plain (SSC6, Chang I2) and deep lacustrine (SSC7, Chang 11) sub-facies can be recognized from the bottom to the top. The deep lacustrine sub-facies is composed of deep lacustrine mudstones and subaqueous turbidity fan sandstones.Mudstones in the MSC9 lacustrine facies, formed in a weak oxidation environment, are of high organic matter with total organic carbons ranging from 1% to 10% and average of 3.5%, average vitrinite reflectance 0.65% and I -Ilkerogen types, which indicate that the organic mater in the mudstones is in its low mature to mature stage and resulted in good source rocks. Sandstones in the MSC8 fluvial system are reservoirs with medium to fine-grained lithic arkose as the main sandstone rock types, secondary dissolution pores developed and with a small amount of micro-fissures which led to better reservoir quality, measured porosity ranges 10%-23% with average 15%, permeability ranges 0.2 * 10~3um2- 120 * 10~3um2 with average 25 * 10~3um2. Whereas, sandstones in subaqueous rurbiditic fan of the MSC9 are with high content of carbonate cement (av. 11%) which resulted in poor reservoir properties (average porosity 15%, average permeability 2 x 10 3|im2). Study also shows that sedimentary facies and diagenesis are the major factors controlling reservoir quality.Research on correlation of source rock provenance indicates that hydrocarbons in the Chang 2 are characterized by mixied source provenances, which may attributed to that source rocks are presented not only in the Chang 1 Member, but also distributed from the Chang 4 to Chang 7 Members. Oils in the Chang 1 reservoir in Ganquan area southern part of the study area may come from the in situ source rocks. Two suites of hydrocarbon pool-forming assemblages can be recognized in the LSC4: pool-forming assemblage of the MSC8 with exotic and mixed sources and pool-forming assemblage of the MSC9 with in situ-sourced and mixed sources, based upon study result of source rock correlation and sandstone types in thedepositional systems. The same characteristics of the two suite is that framework sandstone bodies in the fluvial system have been acted as the major conduit system, while mudstones in the overlie (MSC9) and in the underlie (MSC4-6) lacustrine facies are as excellent source rocks for reservoirs in the fluvial system.Study shows that movement of the Long-long-term Sea Level and Long-term Sea Level during the Late Triassic in the Ordos Basin controlled developing phases and spatial distribution patterns of the major and accessory source rocks and important reservoirs for hydrocarbon accumulation as well. When the Long-long-term Sea Level Cycle (LLSC1, Yanchang Formation) rose to its culmination, a strata containing the major source rocks (MSC4, Chang 7 Member) was developed; whereas when the three Long-term Sea Level Cycles (LSC1, LSC3 and LSC4) went up their maximum levels, respectively, three stratigraphic beds bearing the accessory source rock beds of Chang 9 (MSC2), Chang 4+5 (MSC6) and Chang 1 (MSC9) occurred. By contrast, the three Long-term Sea Levels (LSC1, LSC3 and LSC4) fell to their minimum levels during which depositional environments were relatively stable and the three important hydrocarbon reservoirs of Chang 8 (MSC3), Chang 6 (MSC5) and Chang 2 (MSC8) occurred.Research also implies that abnormal high pressure was the main dynamic drive for primary hydrocarbon migration. Depositional facies, hydrocarbon generation centers and regional nose-like structures are the major factors that control distribution and accumulation of hydrocarbons in pool-forming assemblages.