| Foreland basin, whose development primarily related to the ongoing activities of thrust belts, has been recognized as the basin type with many large oil & gas fields and world’s richest oil & gas. In addition, early and late sedimentation in the typical foreland basin varies greatly. Irrawaddy basin, in which foreland fluvial and delta sedimentary models are extensively developed, is a typical foreland basin and main hydrocarbon producing areas in Myanmar, therefore its oil and gas exploration prospects should be very good. However, due to many factors, Myanmar’s oil and gas exploration degree is relatively low with very poor or extremely limited cognition about tectonic evolution, stratigraphic sequence etc. for major oil and gas basins. Sinopec SIPC has successfully got one right and interest block in Irrawaddy basin, however, following Patolon-1 to obtain oil and gas breakthrough, recent drilling results are extremely poor. Therefore, SIPC desperately needs intensive geological study in order to achieve bigger break in the near future.In view of these, mainly based on six field geological profiles, combined with other materials including literature, laboratory analysis, core, drilling and logging data, we have carried out a lot of systematic research work on basin tectonic evolution, sequence stratigraphy, depositional filling pattern etc. in order to provide guidance for the oil and gas exploration in the future in Irrawaddy foreland basin.By means of analyzing field geological profiles, six typical unconformities have been found. Based on these unconformities, two higher cycles and five tectonic evolution stages have been divided in Cenozoic tectonic evolution history. Compared with regional seismic profiles, three regional and one sub regional unconformity has been identified, then we divided the sedimentary strata since late Cretaceous into four tectonic sequences and fourteen third-order sequences. Based on above research results, we successfully established sequence stratigraphic columns in major sedimentary sags by detailedly described and analyzed formation lithologic information, fossil assemblages etc.Then, within above sequence stratigraphic frameworks, continental, transitional and marine sedimentary system groups have been recognized in the Cenozoic sedimentary strata by analyzing sediment source and identifying sedimentary facies marks such as fossils and sedimentary structures etc. Meanwhile, we firstly proposed that sedimentary system groups have perfect correspondence with proceeding second-order sequences, which confirmed the tectonic sequences’ position. Furthermore, sedimentary system groups can be divided into ten depositional systems like fluvial, river-dominated delta and alluvial fan etc. Based on these understandings, four kinds of sedimentary models including alluvial fan-wave dominated delta, neritic shelf facies, restricted sea- tide dominated delta and foreland fluvial-delta have been creatively summed up.In addition, by aiming at strata with abundant and good quality research data, we have carried out the exploratory study on sedimentary facies. Combined with analysis results of tectonic evolution, we concluded that tectonic evolution has apparent controlling effect on sediment source, framework, cycle and stratigraphic distribution etc. What is more, after comprehensive analysis on tectonic evolution, sequence and sedimentary facies, we not only screened out four sets of main source rock like Laungshe formation of lower Eocene, four sets of core reservoirs like Taybin formation of middle Eocene and three sets of cap rocks like Pyawbew formation of lower Miocene, but also fundamentally identified four kinds of oil and gas plays. Eventually, combined with trap distribution, we pointed out the favorable zone of oil & gas and the next step exploration direction in Irrawaddy basin, which can act as guidance and reference for Sinopec strategic constituencies and exploration & development work in Myanmar. |
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