Geochemistry And Hydrocarbon Generation Of Source Rocks And Origins Of Natural Gases In The Deepwater Area Of The Qiongdongnan Basin

The Qiongdongnan Basin is one of the four major petroleum-bearing basins in the northern part of the South China Sea.Nowadays the petroleum exploration in Qiongdongnan Basin has moved from shallow water areas to deep water areas and several gas pools have been discovered,including LS17,LS18,LS25 gas pools in the central cayonon reservoirs as well as the YL8 buried-hille gas pool in the Songnan Low Uplift.Howerver,there are still some controversial issues regarding their genetic origins and source kitchens of these gas pools and the future exploration potential in the deep water areas.For example,both marine and marine-terrestrial source rocks are present in the deep water area,but it is not clear which one is the main souce rock;furthermore,it is also not clear what is the difference in petroleum generation between the two types of source rocks,which subsequently impedes the genetic correlation between oil and gas and their source rocks.In this study,both types of source rocks were systematically collected from drilling wells in the deep water areas for detailed analysis of organic geochemistry,organic petrology and petroleum generation kinetics to investigate their differences in geochemical character and petroleum generation potential.Kientic parameters were obtained for typical source rocks,based on which the gas origins,source kitchens and accumulation models main gas pools were investigated,which provides important reference for the exploration in the deep water area of the Qiongdongnan Basin.The main conclusions of this study incude:?1?In general,both types of source rocks have low TOC values?< 1%?except that some marine-terrestrial source rocks have TOC values greater than 2%.The marine-terrestrial source rocks have hydrogen index?HI?values ranging from 50 to 200 mg HC/g TOC with macerals dominated by vitrinite and inernite,indicating they are compsed of typical type III kerogen;the marine source rocks have HI values in the range of 250–400 mg HC/g TOC and higher contents of exinites than the marineterrestrial source rocks,implying that they are mainly composed of kerogens of type II2 and III.The stable carbon isotope of organic matter becomes heavier when the content of exnite maceral increases from Yacheng Formation to Lingshui Formation and from marine-terrestrial to marine environments,and the boundary value is about-27‰.The reasons for this abnormal distribution of stable carbon isotope are likely realted to that the marine organisms have utilized the bicarbonate in water whose carbon isotope is heavier than the free CO₂ in water and that the CO₂ concentration during the Lingshui Formation was reduced which led to the enrichement of ¹³C in the C₃ plant.?2?In the pressure range of 50–150 MPa,the increase of pressure generally retardes the gas generation,in particular the generation of C2-5 wet gas.At the same time,an increase in pressure also leads to increased activation energy and frequency factors for gas generation,which may be explained respectively by the greater work during gas expansion at higher pressure and the more extensive molecular collisions in a smaller reaction system.In geological condition,the temperature for entering into gas window increase approximately 10 oC for every increase of 50 MPa at the heating rate of 5 oC /Ma.Moreover,the increased pressure also leads to a lighter methane carbon isotpe at similar thermal maturity levels by retarding methane generation and changing the formation mechanism of methane.?3?By restoring the burial history and thermal history of wells,combing with the formation pressure to optimize the kinetic parameters,the history of methane conversion and carbon isotope fractionation has been restored.The natural gas in the central caynon reservoirs in the Lingshui Sag is of themogeneic origin and mainly derived from the marine source rocks rather than the conventionally believed marineterrestrial source rocks.Among of them,the natural gas in LS25 gas field mainly migrated vertically from the deep source kitchen,whereas lateral migration of gas from the Lingshui Sag formed the LS18 gas field.For the LS17 gas field,there are both vertical and lateral gas migration.The natural gas in YL8 gas field are mixture of thermogenetic and bacterial gases,and the thermogenetic gas is mainly derived from the marine source rocks in the slope areas of Songtao and Baodao sags,which migrated laterally to the traps in the Songnan Low Uplift.?4?The natural gas discovered in the Lingshui Formationa by Well WN1 in Changchang Sag is also of thermogenetic origin and mainly originated from the marine Yacheng source rock.The main reason for the lack of industrial gas pool in Well WN1 is mainly attributed to the poor reservoir quality and the damage of trap by volcanic intursion.Considering that the marine Yacheng source rocks in the slope areas of Changchang Sag are in gas window,there are still promising potential for gas accumulations where high quality reservoirs without volcanic intrusions are developed.