Abstract The Lower Bakken Shale is a key member of the Bakken Petroleum System, which is a prolific unconventional accumulation in North America. Unconventional accumulations have unpredictable lateral variations in hydrocarbon production due to a variety of factors including porosity, permeability, and other rock properties. Therefore, understanding the geochemical (source rock potential), petrological, and petrophysical properties of these units is essential in evaluating the hydrocarbon potential for the Lower Bakken Shale. This study utilized cores from four wells within three fields in Divide County, North Dakota, with samples collected for Rock- Eval pyrolysis, organic petrology, petrographic thin section studies, XRD, SEM, porosity, pore size, and pore fluid distribution. Helium porosimetry and NMR T2 porosity techniques were used to estimate porosity and also to check the quality of the results and avoid discrepancies. Results showed that maturity in the study area varies from immature to early mature using Tmax and solid bitumen reflectance. Organic petrology showed the dominance of solid bitumen and marine alginites, which confirms the Type II kerogen identified from pyrolysis and marine depositional environment in the study area. Major organic matter types identified from SEM studies involve stringy OM, OM-mineral admixture, particulate OM and pure OM which host the majority of the organic matter pores. Furthermore, pore types identified from SEM include mineral matrix pores, organic matter pores, and microfracture pores. Porosity values based on both helium and NMR varies but the difference was nominal and attributed to the presence of abundant clay minerals. Pore sizes are distributed within micropores, mesopores, and macropores with thermal maturity, TOC, and clay mineral proportion having a major influence on pore distribution. Clay-bound water was identified to be the dominant fluid within the shale samples using the T2 cutoff values and supporting evidence from the abundance of clay matrix porosity.

中文翻译：

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北达科他州迪维德县巴肯页岩下部的地球化学、岩石学和岩石物理特征

摘要 下巴肯页岩是巴肯石油系统的重要成员，是北美多产的非常规油气藏。由于包括孔隙度、渗透率和其他岩石特性在内的多种因素，非常规油气藏在油气产量方面具有不可预测的横向变化。因此，了解这些单元的地球化学（烃源岩潜力）、岩石学和岩石物理特性对于评估下巴肯页岩的油气潜力至关重要。这项研究利用了北达科他州迪维德县三个油田的四口井的岩心，收集的样品用于 Rock-Eval 热解、有机岩石学、岩相薄片研究、XRD、SEM、孔隙度、孔径和孔隙流体分布。氦气孔隙率测定法和 NMR T2 孔隙率技术用于估计孔隙率并检查结果的质量并避免差异。结果表明，使用 Tmax 和固体沥青反射率，研究区的成熟度从未成熟到早成熟不等。有机岩石学显示固体沥青和海藻岩占主导地位，这证实了从研究区的热解和海相沉积环境中确定的 II 型干酪根。从 SEM 研究中确定的主要有机质类型包括细丝 OM、OM 矿物掺合物、颗粒 OM 和纯 OM，它们占据了大部分有机质孔隙。此外，通过扫描电镜识别的孔隙类型包括矿物基质孔隙、有机质孔隙和微裂缝孔隙。基于氦气和核磁共振的孔隙度值有所不同，但差异是名义上的，归因于存在丰富的粘土矿物。孔径分布在微孔、中孔和大孔中，热成熟度、TOC 和粘土矿物比例对孔分布有主要影响。使用 T2 截止值和来自粘土基质孔隙度的支持证据，粘土结合水被确定为页岩样品中的主要流体。