Characteristics And Significance Of Dark Series Of Xujiahe Formation In Mabian-muchuan Area

The Xujiahe Formation of Upper Triassic in Sichuan Basin contains abundant tight sandstone gas resources,which is the key direction of oil and gas resources research and has positive significance for exploration and development of unconventional oil and gas resources.Although the predecessors have done a lot of research work on the Xujiahe Formation in southwestern Sichuan,there are few reports on the dark series of the Xujiahe Formation.Dark series are the hydrocarbon-generating horizons of Xujiahe Formation oil and gas resources.Studying the genetic model of dark series is of great help to understand the hydrocarbon-generating process and enrichment horizons of tight sandstone gas in Xujiahe Formation.Based on "4 regional geological surveys projects of Mabian County,Muchuan County,Dianlan Ba and Xinshi Town in Wumeng Mountain Area of Sichuan Province with scaling of 1:50 thousand",this paper has carried out profile measurement,lithological observation,sample collection and geological mapping in the study area,and then carried out thin section identification,principal trace elements analysis,fossil identification,TOC determination,scanning electron microscopy observation and fluorescence X-ray diffraction in the laboratory.On the basis of the above experimental results,sequence stratigraphy,sedimentary petrology,geochemical principles,petroleum geology,Paleontology and other theories are used to study the sequence stratigraphic characteristics,sedimentary facies characteristics and dark stratigraphic characteristics of the Xujiahe Formation in the study area.Finally,the dominant factors of the dark stratigraphic system are discussed and the genetic model of the dark stratigraphic system is determined.Based on the field measured profiles,thin section identification and the comprehensive study of previous data and literature,the Xujiahe Formation in the study area can not be divided into six members in accordance with the adjacent areas due to the poor outcrop situation in the region.The Xujiahe Formation in the study area is divided into two members from bottom to top.The boundaries of the first and second member of the Xujiahe Formation are bounded by the muddy gravel sandstone at the bottom of the second member of the Xujiahe Formation.The first member of the Xujiahe Formation has parallel unconformity contact with the underlying Leikoupo Formation,and the second member of the Xujiahe Formation has integrated contact with the underlying Ziliujing Formation.According to the principle of sequence stratigraphy and the characteristics of sequence identification interface and lithologic assemblage,the sequence of Xujiahe Formation in the study area can be divided into one ultra-long cycle,which can be divided into two long-term cycles and five medium-term cycles.The thickness of sequence LSC1 varies little in the study area.The sequence thickness ranges from 193.7m to 265.2m,with an average of 243.3m.Sequence characteristics show that the thickness of ascending and descending half-cycles is roughly the same,which belongs to symmetrical cycle structure and has a relatively complete rhythmic sea(lake)regression-sea(lake)cycle process.Sequence LSC2 is composed of MSC3,MSC4 and MSC5 in the second member of Xujiahe Formation.It is a relatively complete lacustrine regression transgressive sedimentary cycle.The thickness of sequence LSC2 varies greatly in the study area,ranging from 137.81 m to 592.75 m,with an average of 376.1m.Most of the dark series(coal line and coal seam)developed in Xujiahe Formation can be used as the relative transformation interface from upward to downward,which is equivalent to the condensate layer in marine strata.The sedimentary facies can be divided into continental sedimentary system and transitional marine-continental sedimentary system by distinguishing lithological types,identifying sedimentary structures and paleo-biomarkers.Dark series mainly occur in the lower part of Xu 1,middle and upper part of Xu 1 and middle part of Xu 2.They are mainly developed in delta front subfacies distributary Bay microfacies,delta plain marsh microfacies,shore lake marsh microfacies and shore shallow lake mud microfacies.The dark layer thickness in the study area ranges from 34.7m to 268.3m,with an average thickness of 149.8m.The mineral composition,geochemical element content and TOC content of the samples were determined systematically.The samples were identified by thin sections and observed by scanning electron microscopy.The results show that the average quartz content is 56.38%,clay minerals 36.78%,feldspar minerals 5.79%,and other minerals such as pyrite,calcite and dolomite are about 1-3%.The average brittleness index is 74.0%,which is mainly distributed between 40% and 100%,and has good fracturability.TOC content shows that the dark series belong to medium-better source rocks.Geochemical data show that the sedimentary environment of dark series is freshwater anoxic environment.On the basis of comprehensive analysis of the above results,it is considered that the dark beds are mainly controlled by anoxic reduction environment and sedimentary facies.Oxygen content at the bottom of the water body is low,which belongs to anoxic environment and produces dark layers.It is not easy to oxidize in anoxic environment,which is conducive to the enrichment and preservation of organic matter.The distribution of sedimentary facies controls the distribution of dark beds,which are limited by the range of lake basins and plain marshes.