Source Rock Feature And Oil-Source Correlation Of The Mid-Lower Yanchang Formation In Tiebiancheng Area,Central-Western Ordos Basin

Hydrocarbon origin and accumulation model of the mid-lower part of Yanchang formation in Jiyuan oil field,central-western Ordos basin,have attracted much attention in recent years.Based on multiple analysis results from drilling core observation,well logging interpretation,division and correlation of substrata,sedimentary facies,hydrocarbon source rocks and crude oil,combined with the studying result of the Organic carbon abundance?TOC?obtained by logging interpretation and excess pressure,a systematical research is conducted for the source rock potential,source-reservoir pressure difference,direction of hydrocarbon migration,Hydrocarbon origin and accumulation model of the mid-lower part of Yanchang formation inTiebiancheng area in Jiyuan oil field.The main achievements can be concluded as follows:?1?Core observation and well logging interpretation and sedimentary facies tppes analysis results show that the shallow-deep lacustrine subfacies deposits are mainly developed in Chang 7 oil reservoir of Yanchang formation in the Tiebian area,chang 8 oil reservoir in delta front subfacies deposition and local development of predelta subfacies deposits,chang 9 oil reservoir mainly develops the delta front subfacies deposition and local development of predelta or shallow lacustrine subfacies deposits.?2?Organic geochemical analysis of source rocks,organic carbon abundance logging interpretation?TOC?and thickness of source rocks and TOC mapping analysis results show that in Tiebiancheng area,Chang 7 oil reservoir lithologic including shale and dark mudstone,whose average thickness is 21.6 m and its kerogen type is?-?_1,TOC average is 3.9%,reaching the mature stage,all the factors contribute to Chang7 to be the best hydrocarbon source rocks in the study area;The mudstone thickness of chang 8 oil reservoir is small and discontinuous with low TOC as non-poor level of hydrocarbon source rocks,its kerogen types is?₂-?,reaching the mature stage;The average thickness of dark mudstone source rocks in chang 9 oil reservoir is 5.4m,it's kerogen type is?₂,reaching the mature stage,and its maximum TOC is 1.7%,the average 0.97%with a medium to the level of hydrocarbon source rocks.?3?The calculation and analysis results of excess pressure show that the average excess pressure of Chang7,Chang 8 and Chang 9 oil reservoirs in the study area is 16.8 Mpa,12.2Mpa and 12.3 Mpa.The excess pressure difference between Chang 7and Chang 8 or Chang9 oil reservoirs in the main source rocks is 1.4⁶.8 Mpa?average 4.5 Mpa?and 1.8⁷.4 Mpa?average 4.6 Mpa?,indicating that the driving force of source-reservoir pressure difference exists between Chang 7 and Chang 8 or Chang 9 oil reservoirs.The excess pressure difference between Chang 9 and Chang 8 oil reservoirs in the secondary source rocks is distributed in the range of-1.7².2 Mpa?average 0.1 Mpa?,so it does not have the driving force conditions for Chang 9 source rocks to supply oil to Chang 8 oil reservoir.?4?Oil-source correlation and its comprehensive analysis with the hydrocarbon source rocks and source-reservoir pressure differential driving force conditions show that the crude oil of Chang7 and Chang8 oil reservoirs in the study area comes from Chang7 source rocks;The crude oil of Chang 9 oil reservoir is mainly contributed by Chang7 oil shale,and there is also the contribution of Chang 9 dark mudstone source rocks.The main body of the Chang 8 oil reservoir in the study area is controlled by the"single-source hydrocarbon supply"from the main source rocks of Chang 7 source rocks,while the Chang 9 oil reservoir shows the"Dual source primary and secondary hydrocarbon supply"from the Chang 7 and Chang 9 source rocks.