Sedimentology and Ichnology of Upper Montney Formation Tight Gas Reservoir, Northeastern British Columbia, Western Canada Sedimentary Basin

Several decades of conventional oil and gas production in Western Canada Sedimentary Basin (WCSB) have resulted in maturity of the basin, and attention is shifting to alternative hydrocarbon reservoir system, such as tight gas reservoir of the Montney Formation, which consists of siltstone with subordinate interlaminated very fine-grained sandstone. The Montney Formation resource play is one of Canada’s prime unconventional hydrocarbon reservoir, with reserve estimate in British Columbia (Natural Gas reserve = 271 TCF), Liquefied Natural Gas (LNG = 12,647 million barrels), and oil reserve (29 million barrels). Based on sedimentological and ichnological criteria, five lithofacies associations were identified in the study interval: Lithofacies F-1 (organic rich, wavy to parallel laminated, black colored siltstone);Lithofacies F-2 (very fine-grained sandstone interbedded with siltstone);Lithofacies F-3A (bioturbated silty-sandstone attributed to the Skolithos ichnofacies);Lithofacies F-3B (bioturbated siltstone attributed to Cruziana ichnofacies);Lithofacies F-4 (dolomitic, very fine-grained sandstone);and Lithofacies F-5 (massive siltstone). The depositional environments interpreted for the Montney Formation in the study area are lower shoreface through proximal offshore to distal offshore settings. Rock-Eval data (hydrogen Index and Oxygen Index) shows that Montney sediments contains mostly gas prone Type III/IV with subordinate Type II kerogen, TOC ranges from 0.39 - 3.54 wt% with a rare spike of 10.9 wt% TOC along the Montney/Doig boundary. Vitrinite reflectance data and Tmax show that thermal maturity of the Montney Formation is in the realm of “peak gas” generation window. Despite the economic significance of the Montney unconventional “resource-play”, however, the location and predictability of the best reservoir interval remain conjectural in part because the lithologic variability of the optimum reservoir lithologies has not been adequately characterized. This study presents lithofacies and ichnofacies analyses of the Montney Formation coupled with Rock-Eval geochemistry to interpret the sedimentology, ichnology, and reservoir potential of the Montney Formation tight gas reservoir in Fort St. John study area (T86N, R23W and T74N, R13W), northeastern British Columbia, western Canada.

Geological characteristics and ‘‘sweet area'' evaluation for tight oil

Tight oil has become the focus in exploration and development of unconventional oil in the world, especially in North America and China. In North America, there has been intensive exploration for tight oil in marine. In China, commercial exploration for tight oil in conti- nental sediments is now steadily underway. With the dis- covery of China＇s first tight oil field--Xin＇anbian Oilfield in the Ordos Basin, tight oil has been integrated officially into the category for reserves evaluation. Geologically, tight oil is characterized by distribution in depressions and slopes of basins, extensive, mature, and high-quality source rocks, large-scale reservoir space with micro- and nanopore throat systems, source rocks and reservoirs in close contact and with continuous distribution, and local ＂sweet area.＂ The evaluation of the distribution of tight oil ＂sweet area＂ should focus on relationships between ＂six features.＂ These are source properties, lithology, physical properties, brittleness, hydrocarbon potential, and stress anisotropy. In North America, tight oil prospects are distributed in lamellar shale or marl, where natural fractures are fre- quently present, with TOC 〉 4 %, porosity 〉 7 %, brittle mineral content 〉 50 %, oil saturation of 50 %-80 %, API 〉 35~, and pressure coefficient 〉 1.30. In China, tight oil prospects are distributed in lamellar shale, tight sand- stone, or tight carbonate rocks, with TOC 〉 2 %, poros- ity 〉 8 %, brittle mineral content 〉 40 %, oil saturation of 60 %-90 %, low crude oil viscosity, or high formation pressure. Continental tight oil is pervasive in China and its preliminary estimated technically recoverable resources are about （20-25） × lO8^ t.

An overview of the geology and production of the Fuling shale gas field,Sichuan Basin,China

The Fuling shale gas field in China is the largest shale gas field as well as the largest of its type discovered in any Lower Paleozoic formation.In this study,the geology and production of the upper and lower gas layers in the Fuling shale gas field are evaluated in terms of structure,shale quality,fault,initial production,and estimated ultimate recovery(EUR).The shale in the lower gas layer of the Jiaoshiba anticline is a high-quality reservoir,where the space is dominated by organic pores in kerogen,and the gas content is high.The shale gas wells reveal relatively high initial production and EUR.However,the shale in the upper gas layer of the Jiaoshiba anticline has reservoir space mainly composed of clay mineral pores and organic pores within bitumen,and the gas content is low.In terms of structure,primary gas migration may occur in the upper gas layer,resulting in free gas accumulation in the structural high,where the development effects are generally better than those in the structural low.The lower gas layer in the Pingqiao anticline,is the main interval for shale gas accumulation and development due to the high-quality shale.Under the influence of faults,the efficiency of exploration wells emplaced on top of the anticline is much lower a compared with those in the flanks.The residual synclines close to the Sichuan Basin,including the Baima and Baitao anticlines,are characterized by more recent uplifts,larger area,greater distance from the deep and large faults,and early fracture closure.Therefore,we recommend that the shale gas exploration and development should be carried out preferentially in areas close to the center of the residual synclines,featuring relatively high-pressure coefficient and moderate burial depth.

Isotopes (¹³C and ¹⁸O) Geochemistry of Lower Triassic Montney Formation, Northeastern British Columbia, Western Canada

Oxygen isotope (δ18O) serves as paleothermometer, and provides paleotemperature for carbonates. δ18O signature was used to estimate the temperature of fractionation of dolomite and calcite in Montney Formation, empirically calculated to have precipitated, between approximately 13°C to ±33°C during Triassic time in northeastern British Columbia, Western Canada Sedimentary Basin (WCSB). Measurements of stable isotopes (δ13C and δ18O) fractionation, supported by quantitative X-ray diffraction evidence, and whole-rock geochemical characterization of the Triassic Montney Formation indicates the presence of calcite, dolomite, magnesium, carbon and other elements. Results from isotopic signature obtained from bulk calcite and bulk dolomite from this study indicates depleted δ13CPDB (-2.18‰ to -8.46‰) and depleted δ18OPDB (-3.54‰ to -16.15‰), which is interpreted in relation to oxidation of organic matter during diagenesis. Diagenetic modification of dolomitized very fine-grained, silty-sandstone of the Montney Formation may have occurred in stages of progressive oxidation and reduction reactions involving chemical elements such as Fe, which manifest in mineral form as pyrite, particularly, during early burial diagenesis. Such mineralogical changes evident in this study from petrography and SEM, includes cementation, authigenic quartz overgrowth and mineral replacement involving calcite and dolomite, which are typical of diagenesis. High concentration of chemical elements in the Montney Formation?-Ca and Mg indicates dolomitization. It is interpreted herein, that calcite may have been precipitated into the interstitial pore space of the intergranular matrix of very fine-grained silty-sandstone of the Montney Formation as cement by a complex mechanism resulting in the interlocking of grains.

阜新盆地东梁区浅层天然气成藏特征

研究和评价了阜新盆地东梁区浅层天然气藏的成藏特征,认为气源岩为白垩系沙海组4段泥岩和沙海组3段煤层,巨厚的泥岩和煤层有强大的生烃能力,储层为低孔低渗的沙海组3段砂岩,沙4段泥岩、粉砂岩是区域性良好盖层,沙4段泥岩、沙3段煤层和沙3段砂岩由于渗透率的不同形成了完整的岩性圈闭。东梁背斜沙海组的圈闭面积达10km2,且向东南延伸呈穹隆构造,封闭良好,对天然气形成了一个良好的局部背斜圈闭。研究表明该区沙3段砂岩储层紧邻气源岩,盖层条件良好,局部圈闭,形成“上生下储型”和“自生自储型”的小型浅层气藏。

克拉通盆地致密气成藏地质特征与勘探目标优选——以鄂尔多斯盆地上古生界为例

鄂尔多斯盆地上古生界致密砂岩气为我国目前储量规模最大、产量最高的致密砂岩气藏。气藏大面积、低丰度分布,有利勘探优选困难。通过构造、沉积、地温场等盆地特征和致密气成藏主控因素分析,阐明了鄂尔多斯盆地上古生界致密气成藏地质特征和成藏模式,优选了有利勘探目标。研究认为:克拉通盆地沉积基底平缓、构造稳定、地温梯度较低等盆地属性是决定鄂尔多斯盆地上古生界致密砂岩气生、储、盖大面积叠合,储层先致密后成藏、天然气近距离运聚、大面积低丰度分布的根本原因;致密砂岩天然气差异充注,相对优质储层含气饱和度高。粗粒高石英含量砂岩储层粒度粗、刚性碎屑含量高,压实作用相对稍弱、溶蚀作用相对较强,喉道整体较粗、孔喉连通性好,应力敏感性和水锁伤害相对较弱,储层改造效果好,既是储集物性"甜点"也是储层改造的工程"甜点"。当前储层改造技术水平下相对优质储层分布区的预测是研究区致密气有利勘探开发目标评价的工作核心;由富石英物源控制的石英砂岩区、同物源背景下的强水动力沉积微相区、河道长期稳定发育箱形砂体结构的主河道区等是不同层次的有利勘探目标。