The Organic Matter Accumulation Mechanism And Formation Model Of Chang 9 Black Shales From The Ordos Basin

There are two Mesozoic hydrocarbon source rocks in the Ordos Basin,including Chang 7 and Chang 9 source rocks.The Chang 7 source rocks was regarded as the major hydrocarbon source rocks.Due to the low degree of the exploration and the few data on the deeper sediments of the Yanchang Formation,Chang 9 was ignored and regarded as poor hydrocarbon source rocks.However,in recent years,it has been found that Chang 9black shales contain abundant organic matter（OM）content and good OM type,and they are the source rocks of oil in Chang 8—10 oil group.However,it is still unclear about the distribution,OM accumulation and formation mechanism of Chang 9 black shales.Organic geochemistry,organic petrology and element geochemistry were employed to study the distribution,paleoclimate and sedimentary environments of Chang 9 black shales,define the OM accumulation mechanism of Chang 9 black shales,and reconstruct the development model of Chang 9 black shales.The main results are as follows:The Chang 9 member has two types of hydrocarbon source rocks,including black shales and grey mudstones,and they are very good to excellent source rocks and moderate to good source rocks,respectively.The kerogen type of the Chang 9 black shales and grey mudstones is typeⅠ—Ⅱ₁andⅡ₂—Ⅲ,respectively.The Chang 9 mudstones and shales are mainly in the early oil window to the peak oil window as indicated by R_o,T_max and biomarker thermal maturity parameters.Chang 9 oil group can be divided into Chang 9²and Chang 9¹ mudstones and shales from bottom to top.The Chang 9² black shales are mainly developed around Yingwang—Luochuan areas,and their thickness ranges from 8to 12m;the Chang 9¹ black shales have two depocenters,including the Gaoqiao area in Shanbei Slope with a thickness ranging from 8 to 12m and the eastern Luochuan area,with a maximum thickness of 10m.The source rocks from Chang 9² and the lower part of Chang 9¹ deposited in the delta front are mainly composed of hydrogen-rich vitrinites and bituminites,which were primarily derived from higher plants.The source rocks from the upper part of Chang 9¹deposited in the delta front primarily contain exinite or hydrogen-rich vitrinites,which were dominated by OM from higher plants.The source rocks from the upper part of Chang 9¹ deposited in the semi-deep lake are mainly composed of lamalginites and mineral-bituminous matrix,which were primarily derived from lower aquatic organisms.These results are consistent with the biomarker analyses.Four series of rearranged hopane compounds have been detected and identified in Chang 9 samples,including 18α（H）-neohopane(Ts and C₂₉Ts),17α（H）-diahopane(C₂₉～C₃₅D),early eluting rearranged hopane(C₃₀E)and 21-methyl-28-norhopanes(C₂₉Nsp).Their accumulation has no evident correlation with biological sources,abundance of organic matter and thermal maturity.Oxic environment is beneficial for enrichment of Chang 9 samples mainly from higher plants in shallow water.Zr/Sc vs.Th/Sc diagram and the ternary of 10Al₂O₃-200Ti O₂-Zr indicates that the sediments recycling has an insignificant influence on the Chang 9 mudstones and shales.The provenance of the Chang 9 mudstones and shales is mainly composed of adamellite,with some contribution from granodiorite and granite.The moderate values of the chemical index of alteration（CIA）,the chemical index of weathering（CIW）,and the plagioclase index of alteration（PIA）imply a moderate chemical weathering of the provenance,which is consistent with Rb/Sr,Al₂O₃/Ti O₂,ternarys of A-CN-K and ternarys of AK-C-N.The Sr/Cu,Rb/Sr,C-values and the plot of Si O₂ vs.Al₂O₃+K₂O+Na₂O suggest that the paleoclimate is semihumid,consisting with the moderate chemical weathering.The terrigenous clastic influx is moderate—high during the deposition of Chang 9member,the semi-deep lake area has more moderate terrigenous clastic influx than the delta front and the black shales have more moderate terrigenous clastic influx than the gray mudstones.The biomarker composition,maceral composition and elemental geochemistry indicate that the low paleoproductivity and oxic shallow fresh water was prevailing during the deposition of the mudstones and shales from the Chang 9² and the lower Chang 9¹ in the center of the lake basin（e.g.,Dan 81 well）,the upper Chang 9¹ in the northwest of the lake basin（e.g.,Hu 152 well）and the Chang 9 in the margin of the lake basin（e.g.,Shiwanghe outcrop）;whereas the paleoproductivity was moderate and the fresh bottom water was suboxic and intermittently anoxic during the deposition of black shales from the upper Chang 9¹ in the center of the semi-deep lake（e.g.,Dan 81and Dan 49 wells）.The organic matter accumulation in the semi-deep lake and shallow water environments（delta front）was controlled by the suboxic and intermittently anoxic environments and terrestrial plants influx,respectively.