Study On The Formation Modes Of Coal-measure Source Rocks In Northern Basins Of South China Sea

Coal-measure source rock is one of the most important source rocks in petrliferous basin which is formed in the transitional environment, and the major organic matter of which is derived from higher plants. There are three types of wide-distributed coal-measure source rock series occurred from late Eocene to early Oligocene in Northern basins of South China Sea (NBSCS) including coal-measure source rocks of shallow lake-limnetic facies of Enping Formation (Fm) in Zhu Ⅰ Depression(ZYD) and Zhu Ⅲ Depression (ZSD), Pearl River Mouth Basin(PRMB), coal-measure source rock of littoral-delta facies of Enping Fm in Zhu Ⅱ Depression(ZRD), and coal-measure source rock of littoral-marsh facies of Yacheng Fm in YingQiong Basin(YQB). Differences among source rock geochemical features are derived from the distinctiveness in tectonic background, paleoclimate, sedimentation-subsidence matching relationships, organic matter source and redox conditions of sedimentary environment.The dissertation analyze the geochemical difference of coal-measure source rocks of different sedimentary facies and the formation modes of source rocks based on the reconstruction of paleoclimate, tectonic background, sedimentation-subsidence feature, organic matter supply and redox condition of sedimentary environment via organic geochemical data, seismic data, well data and geobiological data. Four conclusions are come to as follows.(1)Coal-measure source rocks of shallow lake-limnetic facies of Enping Fm in ZYD and ZSD comprises coal, carbonaceous mudstone and middle-poor mudstone in terms of lithology. Organic matter type of source rock is Ⅱ and Ⅲ, and the organic matter is of low-middle maturity.The percentage of regular sterane C29 is larger than C27, and the content of C304-methyl sterane is low. The proportion of C22-C26 long chain tricyclene is larger than C19-C21; Coal-measure source rocks of littoral-delta facies of Enping Fm in ZRD comprises coal, carbonaceous mudstone and middle-good mudstone in terms of lithology. Organic matter type of source rock is Ⅱ2 and Ⅲ, and the organic matter is of middle-high maturity. The percentage of regular sterane C29 is larger than C27, and the content of C304-methyl sterane is low. The proportion of C22-C26 long chain tricyclene is ralativly smaller than C19～C21.Pr/Ph values of Enping Fm in PRMB are distributed between 0.58 and 9.07.Coal-measure source rock of littoral-marsh facies of Yacheng Fm in YQB comprises poor-good mudstone in terms of lithology. The content of long chain tricyclic terpene C19～C21 is similar to C22-C26. Pr/Ph is distributed between 0.23 and 6.09.(2)According to the organic matter abundance, type and maturity, hydrocarbon-rich sags are classified into four groups. Coal-measure source rocks in the first group including Huizhou sag and Baiyun sag in PRMB are of high organic matter abundance and large apparent thickness of mature source rocks. Coal-measure source rocks in the second group including Xijiang sag of PRMB and Yanan sag of QDB are of medium organic matter abundance and medium apparent thickness of mature source rocks. Coal-measure source rocks in the third group including Lufeng sag and Enping sag of PRMB and Yingzhong sag of YQB has compensate relationship between organic matter abundance and maturity, i.e. source rock is of low organic matter abundance and large apparent thickness of mature source rock (Yingzhong sag), or source rock is of medium organic abundance and small apparent thickness of mature source rock (Lufeng sag and Enping sag). Coal-measure source rock of fourth group (Wenchang sag of PRMB) is of low organic matter abundance and medium apparent thickness of mature source rocks.(3)Tectonic background and paleoclimate controls the occurrence of source rocks via influencing the feature of sedimentary environment. Hydrocarbon-rich sags of high organic matter abundance and organic matter maturity in NBSCS has four characteristics:(1) good symmetry, (2) the ratio of long axis to short axis is between 2 and 4, (3) the maximum thickness of coal measure strata is between 1700 meters and 4000 meters, and (4) the area of the sag is between 0.1×104km2 and 4.5×104km2. Coal-measure source rocks are formed in the condition of steadily subsidence, and the tectonic intensity of PRMB is lower than YQB. The paleoclimate is humid and warm during the occurrence of source rocks. The first group of sags are of good symmetry, middle ratio of long axis to short axis (2-3), large maximum thickness of coal measure strata (1700-2750m) and large area (1～2×104km2), and the tectonic intensity during the occurrence of source rock is between 0.25 and 0.43 with tectonic subsidence rate lying between 21 and 107m/Ma. The second group of sags are of good symmetry, middle ratio of long axis to short axis (1.7-2.3), large maximum thickness of coal measure strata (1430-3750m) and middle area (0.1～0.5×104km2), and the tectonic intensity during the occurrence of source rock is between 0.34 and 0.42 with tectonic subsidence rate lying between 52 and 185m/Ma. The third group of sags are of medium symmetry, middle ratio of long axis to short axis (1.5～3), large maximum thickness of coal measure strata (1480-3930m) and medium area (0.5-4.5×104km2), and the tectonic intensity during the occurrence of source rock is between 0.30 and 0.40 with tectonic subsidence rate lying between 33 and 183m/Ma. The fourth group of sags are of medium symmetry, large ratio of long axis to short axis (6), small maximum thickness of coal measure strata (1250m) and medium area (0.4×104km2), and the tectonic intensity during the occurrence of source rock is 0.34 with tectonic subsidence rate of 81m/Ma.(4)The occurrence of coal-measure source rocks is controlled by organic matter supply, redox condition of sedimentary environment and matching relationship between sedimentation and subsidence. During the period of the occurrence of coal-measure source rocks, the PRMB belongs to subtropical to tropical climate and the major plant species is evergeen broad-leaf plants and herbaceous plants. The main organic matter source is higher plants and algae. The mudstone are formed in weak reducing to weak oxidizing environment and coal& carbonaceous mudstone are formed in weak oxidizing to oxidizing environment.On the basis of "Sedimentation rate/Tectonic subsidence rate mode (S/T-TOC mode)" and "Lithological composition explanation modes for coal-measure source rocks", three periods for the occurrence of coal-source rocks are determined:(1) low S/T period (i.e. S/T value of sag margin lies between 1.64 and 2.20, S/T value of sag center of PRMB is lower than 2.71 and S/T value of sag center of YQB is between 3.19 and 3.88), the organic matter abundance of mudstone is small, and the percentage of middle-good source rocks are all small in both PRMB and YQB. The proportion of coal in PRMB is small which might be large in YQB. (2) Middle S/T period (i.e. S/T value of sag margin lies between 2.20 and 2.53, S/T value of sag center of PRMB is from 2.71 to 4.30 and S/T value of sag center of YQB is between 3.88 and 4.82), the organic matter abundance of mudstone is large, and the percentage of middle-good source rocks are all big in both PRMB and YQB while the proportion of coal in PRMB is medium. (3) Large S/T period (i.e. S/T value of sag margin lies between 2.53 and 3.91, S/T value of sag center of PRMB is from 4.30 to 6.60 and S/T value of sag center of YQB is larger than 4.82), the organic matter abundance of mudstone is large, and the percentage of middle-good source rocks are all small in both PRMB and YQB while the proportion of coal in PRMB is high.