The types and quality of source rocks in the Shahezi Formation are the key factors affecting the distributions of various deep gas reservoirs in the Xujiaweizi fault depression in Songliao Basin. To clarify the quality differences and origins of different types of source rocks in the Shahezi Formation, this study reconstructed the sedimentary and water environment, determined the controlling effects of fault activity, sedimentary facies, and paleo-sedimentary environment on the quality of various source rocks, by making full use of seismic, logging, core, organic geochemical and element geochemical analysis. The results show that two types of source rocks developed in the Shahezi Formation, namely, mudstones and coals. The mudstones have a relatively high abundance of organic matter, which consists of type-II kerogen and partial type-III kerogen, and are concentrated in Sha-I Member. The coals have a high abundance of organic matter, which consist of type-III kerogen, and are mainly distributed in Sha-II Member. During the deposition of Sha-I Member, intense fault activity formed arrow, deep-water lacustrine basins with high salinity and strong reducibility on the downthrow sides of faults. During the deposition of Sha-II Member, fault activity progressively weakened, and the areas of lacustrine basins enlarged to their maximum values and became wide, shallow-water basins with low salinity and low reducibility. The development of source rocks was controlled by fault activity, sedimentary facies, and paleo-sedimentary environment. Fault activity formed accommodation space on the downthrown sides of faults for mudstone accumulation, thus determining mudstone thickness. The sedimentary environment controlled the organic matter input and determined the distribution of mudstones and coals. The paleo-sedimentary environment, which consisted of paleo-salinity, as well as paleo-water depth and redox conditions, affected the accumulation and preservation of organic matter and is the main controlling factor for the quality difference of various source rocks in the Shahezi Formation.
A major problem in evaluation of gas hydrate resources is lack of a reliable technique to constrain the top occurrence of gas hydrates. We have developed a numerical model of the top occurrence of gas hydrates based on that the depleted quantity of sulfate diffusion downward equals to that of methane flux upward at sulfate-methane interface (SMI) and the methane solubility in the water-hydrate system, using steady state and analytical expressions. In situ methane production and sulfate depletion associated with the oxidation of sedimentary organic matter were considered in the model. We applied this model to ODP site 1245 and IODP site 1327, and calculated the top of gas hydrate occurrence at these two sites, yielding 53 mbsf, 83 mbsf, respectively, which are consistent with the top depth estimated by drilling.
鲕粒不仅是古环境的重要指示标志,由其构成的鲕粒石灰岩通常也是重要的油气储集层.但自鲕粒被发现一个多世纪以来,关于其无机成因还是有机成因就一直存在争议.分别系统归纳了典型鲕粒的物理化学成因及微生物成因、鲕粒的圈层结构类型及成因,结合前人研究成果建立了新的鲕粒成因模式,解释了常见鲕粒的形成过程.对鲕粒研究综述和实例分析认为,鲕粒新圈层的形成是微生物介导的,鲕粒圈层形态是受沉积水体能量控制的,光合微生物及硫酸盐还原细菌等建设性微生物群落均促进了鲕粒圈层的沉淀;鲕粒的圈层结构主要为径向生长的放射状、随机分布的杂乱状和切线状,在地质历史时期观察到的其他鲕粒结构均是受活跃的破坏性钻孔生物及不同程度成岩作用改造的结果;鲕粒最原始的结构是径向生长的放射状,是在低能静水环境中由非晶质钙碳酸盐(ACC)矿化形成的,随机分布结构及切线状结构是被水动力剪切改造形成的结构,鲕粒中明显的暗色同心薄层是鲕粒生长过程中间歇性出现的微生物群落建设性或破坏性定殖后生物矿化的产物,受水动力影响这三种结构既可以单独发育,也可以组合出现,构成了鲕粒形态的多样性;核心供给速率大小、沉积水体碳酸钙饱和度高低、沉积水体能量强弱及建设性微生物群落活跃程度共同控制了鲕粒粒径大小.
通过对库鲁克塔格地区下寒武统泥页岩的主微量元素数据进行地球化学分析,探讨了早寒武世塔里木盆地东北缘的古气候演化特征、物源输入背景、泥页岩成因模式.CIA值及SiO2-(Al2O3+K2O+Na2O)图解表明,研究区古气候由埃迪卡拉纪进入寒武纪变暖,进入寒武纪后再次变得寒冷干旱.Ce/Ce*值和(La/Ce)N值表明西山布拉克组沉积背景为洋中脊附近的深海洋盆,其上覆的西大山组和下伏的汉格尔乔克组为稳定的大陆边缘背景.La-Th-Sc图解表明恰克马克铁什Ⅰ号剖面泥页岩形成时构造环境为被动大陆边缘,恰克马克铁什Ⅱ号剖面构造环境变化较大.Th-Sc、Th/Sc-La/Sc图解及公式化判别函数表明,恰克马克铁什Ⅰ号剖面母岩性质为中性—中酸性岩浆岩,其物源可能来自于中央隆起带前寒武系的中酸性喷出岩、花岗闪长岩、橄榄石花岗岩等,恰克马克铁什Ⅱ号剖面母岩性质为基性—中酸性岩浆岩,其铁镁质组分明显增加,除中央隆起带物源的输入外,可能受到海底铁镁质热液喷出的影响.
鄂尔多斯盆地西缘石炭系羊虎沟组具有较大的油气资源勘探潜力,但受其复杂的构造活动影响,其从源到汇充填过程及古地理格局尚有诸多争议.通过轻矿物、重矿物、岩屑组合和稀土元素等特征进行物源分析,结合前人研究资料,共划分出西北部阿拉善、东北部阴山、西南部北祁连和东南部北秦岭4个源—汇系统.阿拉善源—汇系统源区构造背景整体以主动大陆边缘和大陆岛弧为主,沉积物主要来自平均上地壳长英质源和镁铁质火成物源区;阴山源—汇系统源区构造背景整体以被动大陆边缘为主,沉积物来源较为复杂,为平均上地壳长英质源、中性岩火成物源区和镁铁质火成物源区的混合;北祁连源—汇系统源区构造背景为被动大陆边缘和主动大陆边缘的混合,沉积物主要源自镁铁质火成物源区和中性岩火成物源区;北秦岭源—汇系统源区构造背景为被动大陆边缘,沉积物主要来自镁铁质火成物源区.4个源—汇系统母岩主要为碱性玄武岩、花岗岩与钙质泥质沉积岩的混合.基于源—汇系统分区,重建古地理格局:(1)阿拉善源—汇系统碎屑沉积物从西北部汇入,向南东方向搬运,且搬运距离较远,发育海陆过渡相的河控三角洲沉积体系;(2)源自阴山源—汇系统的沉积物由东北部汇入,向南西方向搬运,搬运距离远分布面积广,发育海陆过渡相的潮控三角洲沉积体系;(3)北祁连源—汇系统碎屑沉积物由西南部汇入,沿岸发育小型河控三角洲沉积,深水区发育以浅海—半深海陆棚沉积为主的海相沉积体系;(4)北秦岭源—汇系统碎屑沉积物由东南部汇入,沉积砂体受中央古隆起影响搬运距离短,发育海相的障壁岛海岸沉积体系.
