陆相页岩层系岩相类型、组合特征及其油气勘探意义——以四川盆地中下侏罗统为例

页岩层系岩相类型的精细识别与划分是页岩气勘探开发潜力评价的重要基础,但目前关于页岩岩相的识别划分标准及分类方案尚未达成共识。为此,以四川盆地中下侏罗统陆相页岩层系为研究对象,利用岩心观察、全岩矿物X射线衍射分析、薄片鉴定、总有机碳含量(TOC)及氦气孔隙度测试等多种手段,在页岩全岩矿物组成及特征分析的基础上,建立岩相划分方法,开展页岩层系岩相类型识别与划分,研究不同尺度下岩相组合特征,并探讨其对页岩气勘探的意义。研究结果表明:①利用新建立的全岩矿物分区—TOC分级—矿物结构与沉积构造校正及完善的3步岩相划分方法,在该区陆相页岩层系共识别出6类20种页岩岩相类型,其中以中—高碳黏土质页岩岩相、纹层—薄层状黏土质页岩岩相及低—中碳粉砂质页岩岩相为主,次为低—中碳黏土质介壳灰质页岩岩相、含—低碳粉砂质黏土质页岩岩相;②黏土质页岩岩相和介壳灰质黏土质页岩岩相的平均TOC、平均孔隙度均高于粉砂质页岩岩相和粉砂质黏土质页岩岩相,页岩矿物成分及岩相类型对于页岩气源、储性能具有一定的影响;③该区陆相页岩层系可识别出泥灰沉积、泥砂沉积及泥灰砂混合沉积等3类组合,依次体现了湖泊相环境下远源区、近源区、过渡区的沉积特点,对于不同岩相组合的刻画有助于判识页岩沉积环境的差异;④对于高黏土矿物含量的陆相富有机质页岩中灰质介壳纹层与薄层的数量、频次的识别与统计,可以为富气层段储层可改造性评价、最佳勘探开发层段优选提供依据。

Lithofacies types and assemblage features of continental shale strata and their significance for shale gas exploration: A case study of the Middle and Lower Jurassic strata in the Sichuan Basin

Fine identification and division of lithofacies types of continental shale strata is an important basis for the evaluation of shale gas exploration and development potential. At present, however, there is no consensus on the identification standard and division scheme of shale lithofacies. Taking the continental shale strata of Middle and Lower Jurassic in the Sichuan Basin as an example, this paper established a lithofacies division method by means of core observation, whole-rock mineral X-ray diffraction analysis, thin section analysis, total organic carbon (TOC) measurement and helium porosity measurement after analyzing whole-rock mineral composition and shale characteristics. Then lithofacies types of shale strata were identified and divided, and characteristics of lithofacies assemblages in different scales were investigated. Finally, their significance for shale gas exploration was discussed. And the following research results were obtained. First, 20 shale lithofacies types of 6 categories are totally identified in this continental shale strata using the newly established three-step lithofacies division method (whole-rock mineral composition partition–TOC classification–correction and improvement of mineral structure and sedimentary structure). And among them, medium–high TOC clay shale lithofacies, lamina–thin layer clay shale lithofacies and low–medium TOC silty shale lithofacies are dominant, followed by low–medium TOC shell limy clay shale lithofacies, and TOC bearing and low TOC silty clay shale lithofacies. Second, the average TOC and the porosity of clay shale lithofacies and shell limestone clay shale lithofacies are higher than those of silty and silty clay shale lithofacies. It is indicated that mineral composition and lithofacies types of shale have a certain impact on gas source and reservoir performance. Third, three assemblages are identified in the continental shale strata, including mudstone–limestone assemblage, mudstone–sandstone assemblage and mudstone–limestone–sandstone assemblage, which reflect the sedimentary characteristics of distal region, proximal region and transition region in the lacustrine environment, respectively; and that the characterization of different lithofacies assemblages is conducive to recognizing the differences between different shale sedimentary environments. Fourth, fine identification and statistic of the number and frequency of limy shell laminae and thin layers in the terrestrial organic rich shale with a high clay mineral content can provide a basis for the fracturability evaluation of gas rich zones and the selection of optimal exploration and development intervals.