华北地台中-新元古界烃源岩沉积特征及生烃潜力

随着近年来元古宇商业性原生油气藏在全球范围的不断发现,元古宙地层已逐渐成为了油气勘探重点。华北地台燕山地区是中国中—新元古界最为发育的地区之一,中—新元古界总厚可达9553 m,虽油苗等油气显示发现已久,但多年来一直未取得油气突破。为了下一步开展更具针对性的油气调查及勘探,从中、新元古代这套巨厚的沉积地层中优选出油气勘探的有利烃源岩层段就有着非常重要的意义。本文分析了华北中元古代地层中发育的暗色泥岩与微生物碳酸盐岩这两类烃源岩层的沉积特征,并初步评价了它们的生烃潜力。串岭沟组、洪水庄组与下马岭组均发育有厚层的暗色泥、页岩,总有机碳含量TOC平均值分别为0.89%、2.54%、2.82%,有机质的镜质体反射率Ro(采用为镜状体反射率Ro_(vl))值分别为2.03%、1.05%、0.63%。微生物碳酸盐岩在高于庄组、雾迷山组及铁岭组中普遍发育,其内富含微生物群落等丰富的有机质残留,发育TOC>0.2%的优质烃源岩层段。

Organic carbon isotope gradient and ocean stratification across the late Ediacaran-Early Cambrian Yangtze Platform

Organic carbon isotope （δ-13 Corg） data from two well-preserved sections across a shallow-to-deep water transect of the late Edi- acaran-Early Cambrian Yangtze Platform in South China show significant temporal and spatial variations. In the shallow-water Jiulongwan-Jijiapo section, δ-13Corg values of the late Ediacaran Dengying Formation range from -29‰ to -24%0. In the deep-water Longbizui section, δ-13Corg values from time-equivalent strata of the Dengying Formation are mostly between -35‰ and -32‰）. These new data, in combination with δ-13Corg data reported from other sections in South China, reveal a 6‰-8‰ shallow-to-deep water δ-13Corg gradient. High δ-13Corg values （〉-30‰） occur mostly in shallow-water carbonate rocks, whereas low δ-13Corg values （〈-32‰） dominate the deep-water black shale and chert. The large temporal and spatial δ-13Corg variations imply limited buffering effect from a large dissolved organic carbon （DOC） reservoir that was inferred to have existed in Edi- acaran-Early Cambrian oceans. Instead, δ-13C-org variations between platform and basin sections are more likely caused by dif- ferential microbial biomass contribution to total organic matter. High δ-13Corg values （〉-30‰） documented from shallow-water carbonates are within the range of typical Phanerozoic δ-13Corg data and may record the isotope signature of organic matter from primary （photosynthetic） production. In contrast, low δ-13Corg values （〈-32‰） from deep-water sections may have resulted from higher chemoautotrophic or methanotrophic biomass contribution to bulk organic matter in anoxic environments. The δ-13Corg data provide indirect evidence for ocean stratification and episodic chemocline fluctuations in the Ediacaran-Early Cambrian Yangtze Platform.

Environment controls on Mesoproterozoic thrombolite morphogenesis:A case study from the North China Platform

Thrombolites are widespread in the Mesoproterozoic Wumishan Formation in the North China Platform.This study shows that they mainly concentrated in subtidal carbonate facies with relatively low hydrodynamic conditions,rather than in intertidal zone as suggested previously.From the deep lower to the shallow upper subtidal facies,the thrombolites show evident changes in morphology from dominantly domal to tabular forms,likely suggestive of environmental controls on their morphogenesis and distribution.As the most important component in thrombolites,mesoclots typically consist of organic-rich micritic nuclei and organic-poor fibrous aragonite rims.Mesoclots may vary considerably in their morphology,but a type of specifically shaped mesoclots tends to concentrate predominantly in a particular group of thrombolites.The proportion of the fibrous aragonite rims in mesoclots decrease as the depositional environments become shallower,likely suggesting that the environmental changes also have controls on the internal fabrics of thrombolites.Putative filamentous bacterial colonies are well preserved in some aragonite fans in the matrix between mesoclots,invoking rapid precipitation and aragonite-supersaturated conditions in the ambient waters.It seems that a suboxic to anoxic environment,highly alkaline seawater and relatively low hydrodynamic conditions were among the important factors that facilitated the development and preservation of thrombolites in the Mesoproterozoic epeiric sea on the North China Platform.