A Research On Shallow-water Braided Delta’s Sedimentary Organic Facies Of Mungaroo Formation In North Carnarvon Basin, Australia

North Carnarvon basin is located south of Northwest Shelf of Australia, having an area of approximately535000km2. It is a hydrocarbon-rich basin and underwent continuous subsidence during the late Paleozoic-Cenozoic. Multiple world-class gas fields have been found in this basin. Mungaroo Formation, the main hydrocarbon-bearing strata, consists of thick sediments of shallow-water delta controlled by Circum-Tethys mega-monsoon. The distribution of Terrigenous Organic Matter (TOM) in this kind of delta differentiates from that of other deltas without relationship to mega-monsoon event. Mungaroo delta is a good example to study the characteristics of TOM in the delta controlled by mega-monsoon event.Under the influence of mega-mansoon, Mungaroo Formation is notable for the followings:1) well widespread delta plain and narrow delta front and prodelta belts;2) thick channel sandstone frequently sandwich with mudstone which is rich of TOM;3) hygrophytic molecule coexists with xerophytic molecule in sediments.Based on the analysis of well-to-seismic tie, Mungaroo delta can be divided into four sedimentary sub-facies:proximal delta plain, distal delta plain, delta front and prodelta. Using intergrated analysis of geochemistry and organic petrology of source-rocks and palynological assemblage types, the source rocks of Mungaroo Formation are found to be rich in TOM content. The average organic carbon can be up to2.2%. Inertinite in the organic macerals is generally high with an average content of48%, while vitrinite is generally low, having an average content of only38%. The ratio of vitrinite and inertinite of the samples up to72%is less than1, and the ratio of vitrinite and inertinite of all samples are less than3. Peat swamp suffered erosion and caused dispersion of organic matter, while thin coal seam could be developed in the distal delta plain. In additional, organic matter distribution in four sedimentary sub-facies has distinct differences:1) for the proximal delta plain, sandy conglomerate rocks developed with intercalations of thin layers of mudstone. The average organic-carbon content is1.17%and the main maceral in the mudstone is inertinite as most of the vitrinite suffered oxidation;2) peat swamp, characterized by multiple thin coal seams, is well developed in distal delta plain. The mudstone is rich in TOM that average organic-carbon content can be up to2.73%. The vitrinite in distal delta plain is higher than that in proximal delta plain and prodelta;3) the content of TOM in mudstone is low within small-scale delta front and prodelta sub-facies belts. The average organic-carbon content is1.31%and the content of exinite is higher than that of dinoflagellates. In generally, the organic-carbon content gradually decreases from distal plain delta to proximal plain delta (landward) and delta front and prodelta (basinward).Both Monsoon floods and storm currents have an important effect on the distribution of TOM. TOM in the proximal delta plain is easily destroyed and oxidized by strong floods, whereas TOM in the delta front-prodelta is diluted and dispersed by waves and currents. However, the TOM in the distal delta plain could be saved with weaker influence of monsoon floods, waves, and currents. Proposed TOM dispersal patterns should serve as a useful reference for potential reservoir and source rocks in similar deltas around the world.To precisely divide and predict the potential source rocks developed zone of Mungaroo formation, the paper is base on lithological association combine with geochemistry and organic petrology to divide sedimentary organic facies. The depositional environment of different lithological association has difference which also affects the preservation condition of organic matter. Thus the geochemistry and organic petrology also have difference. Based on this concept, the sedimentary organic facies of Mungaroo delta can subdivide into7organic facies:proximal delta plain organic facies (PDP organic facies), rich channel sandstone section in distal delta plain organic facies (DDP1organic facies), rich siltstone section in distal delta plain organic facies (DDP2organic facies), rich coal seams section in distal delta plain organic facies (DDP3organic facies), rich channel sandstone section in delta front (DF1organic facies), rich siltstone section in delta front (DF2organic facies), littoral-neritic and carbonate platform organic facies (LN-CP organic facies). Among them, the hydrocarbon potential of DDP3organic facies are the highest, followed by DDP2, DF2, DDP1, PDP, LN-CP, DF1organic facies. Through the characteristics of the seismic response of different organic facies, the distribution of sedimentary organic facies can be described. Based on the distribution, the exploration potential level of North Carnarvon basin can be subdivided into5areas:I, II, III, IV, and V exploration area. I and Ⅱexploration area is favorable exploration area, which includes distribution area of DDP2, DDP3organic facies. Comprehensive analysis shows that Exmouth plateau is the most favorable exploration area.