Lithology And Sedimentary Facies Of Lower Cretaceous, Tamtsag Basin, Mongolia

South Tamtsag depression is the biggest oil and gas depression in the Tamtsag basin, which has a potential of exploration and development on oil and gas. At the present time, the exploration of south Tamtsag depression is goning on, with 534 exploring wells, 184 oil wells and 8 gas wells in the industrial vaulues. However, the lithology of this region is very complex. This has made the logging interpretation and the reconstruction of sedimentary environments difficult and uncertain, affecting the following exploration. So it is necessary to made exactly the lithology identification and do systemic sedimentary research, to provide advantageous exploration belts and targets.1. MethoLogy of the ResearchBased on the description of well cores, the identification of the thin section and the grain size analysis, we congnised the lithologies of the research area and analyzed sedimentary facies in the wells, while the description of sedimentary features and facies types had been done with the precision to microfacies. Further, with the analysis of the map of sand rate isoline, the preponderant facies was defined to realize the plane distribution feature of sedimentary facies and to analyze the features of sedimentary facies and the directions of provenances.On the above researches and the related data, some analysis of reservoir rocks were given through doing some statistics on the oil and gas shows in the studied wells, to find out the relation between oil and gas shows and sedimentary facies, especially microfacies for the prediction of advantageous reservoir sands.2.Research on Types of RockThe lithology in south Tamtsag depression of Tamtsag Basin is a set of rocks between volcanic lava and normal sedimentary clastic rock, mainly of volcanic lava, pyroclastic rock, sedimentary pyroclastic rock, pyroclastic sedimentary rock, normal sedimentary rock. In the lithology related with volcanic action, abundance of pyroclastic sedimentary occupies the absolute superiority: about 73.24%. The second is pyroclastic rock (19.61%); sedimentary pyroclastic rock occupies 6.84%. Volcanic lava occupies 0.34%, which is the minimum.3.Particularity in The Growth of Sedimentary Facies and The Types of Sedimentary Facies, Sequence The research area in that time was located in a part of volcanism– deposition region and deposition region. There are those seriously mixed of volcanic clasts and normal terrigenous clasts. Volcanic rock exists in forms of sedimentary microfacies in normal sedimentary backgrounds.The particularity of pyroclastics mainly displays in: braided channel of alluvial fan formed by pyroclastic flow, volcanic clast delta in shore-shallow lake.There are four types of sedimentary facies in the target layers: alluvial fan facies, fan delta facies, delta facies, lacustrine facies. Shallow lake subfacies and deep lake subfacies are developed in lacustrine facies. Sometimes in these two subfacies turbidity current and subaqueous fan can develope.Influenced by volcanic activities, lithology sequence composed of conglomerate - pyroclastic rocks/sandstone - mudstone (possibly exist pyroclastic rocks) - sandstone (possibly exist conglomerate), displayed in cycles.4.The Plane Distribution of Sedimentary FaciesThe provenance of south Tamtsag depression prograded to the center of lake from south-west, north and north-east during the formation of Tongbomiao, and alluvial fan, fan delta-delta and shallow lake developed.With the tectonic movement, the faulted basin started to subsidence in period of Tongbomiao.The sedimentary facies belts in Nantun formation distributed in north-east, forming alluvial fan, fan delta, shallow lake and deep lake. The shallow lake was main part, which had the provenance from north-west and north-east. During that period, denudation reduced and settling velocity became faster, entering to the middle of faulted period.. In Damoguaihe formation, delta, shallow lake and deep lake sediments developed in the basin The provenance came from north-northeast. The basin developed to subsidence period, ending the faulted period.5. Distribution of Advantageous Reservoir RocksThe statistical result shows that, the oil and gas in the research area concentrates much on the fan-delta and delta firstly, on the alluvial fan secondly, and on lacustrine sediments lastly. The advantageous reservoir rocks are normal sedimentary rock, sedimentary pyroclastic rock, pyroclastic rock and pyroclastic sedimentary rock in sequence. The most advantageous sedimentary microfacies are distributary mouth bar and subaqueous of delta front, distributary mouth bar and subaqueous of fan-delta front, pyroclastic flow and mudflow of alluvial fan.