黔北下志留统龙马溪组黑色泥岩沉积环境及页岩气成藏条件

下志留统龙马溪组发育黑色泥岩,分布在贵州北部。为阐明其沉积环境和页岩气成藏条件,研究分析了沉积特征和地球化学特征。泥岩具粉砂泥质结构或泥质结构,岩层多为薄层,普遍具水平层理,含浸染状黄铁矿颗粒,表明沉积水动力弱、沉积速率较低。泥岩的Mo、V、U的富集系数都大于1,Ce异常的平均值为-0.033,特定元素比值(V/Cr、Ni/Co、V/(V+Ni)和Ce/La)都显示泥岩形成于缺氧环境。总体上,龙马溪组黑色泥岩的沉积环境为水体较深、沉积速率较低、呈缺氧还原状态。龙马溪组黑色泥岩的TOC含量在2.78%～4.36%之间,平均值为3.79%;镜质体反射率(R_o)在1.88%～2.16%之间,平均值为1.99%,反映有机质已达成熟-过成熟的干气阶段;有效厚度在10～100m之间,脆性矿物含量一般在20%～30%之间,埋深大概在0～3500m。地层厚度从南至北逐渐增大,其余参数各剖面相似。因此,具有页岩气资源潜力的区域大致在桐梓-绥阳以北地区。这些特征综合表明龙马溪组黑色泥岩具备形成页岩气藏的良好条件,存在较高的页岩气资源潜力。

Black Mudstone Sedimentary Environment and Shale Gas Accumulation Conditions of Lower Silurian Longmaxi Formation in Northern Guizhou Province

Black mudstone was developed in the Lower Silurian Longmaxi Formation, which is distributed in northern Guizhou province. In order to clarify the sedimentary environment and shale gas accumulation conditions, the sedimentary and geochemical characteristics were studied and analyzed. The results showed that the mudstone has a silt structure or mud structure; the terranes are mostly thin layer and generally have horizontal bedding. Meanwhile, the mudstone contains the impregnated pyrite granule. These characteristics show that the hydrodynamic force was weak and the deposition rate was low when deposited. The enrichment coefficients of Mo, V and U of mudstone are all greater than 1. The mean Ce anomaly is -0.033. And the ratios of specific elements (V/Cr, Ni/Co, V/(V+Ni) and Ce/La) indicate that mudstone were formed in anoxic environment. On the whole, the sedimentary environment of black mudstone of Longmaxi Formation is characterized by relatively deep water, low deposition rate and hypoxic reduction state. TOC content of black mudstone of Longmaxi Formation varies from 2.78% to 4.36%, with an average of 3.79%. The vitrinite reflectance (Ro) varies from 1.88% to 2.16%, with an average of 1.99%, indicating that organic matter has reached the dry gas stage from mature to over-mature. The effective thickness is between 10m and 100m, the content of brittle minerals is generally between 20% and 30%, and the buried depth is about 0-3 500 m. The thickness of the strata increases gradually from south to north, and other parameters are similar in each profile. Therefore, the area to the north of Tongzi county and Suiyang County in Northern Guizhou has shale gas resource potential. These characteristics indicate that the black mudstone of Longmaxi Formation have good conditions for forming shale gas reservoir and have high shale gas resource potential.