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Shale reservoir has been focused among unconventional resources since the first extraction of free and adsorbed gas from the low-permeable Barnett Shale via horizontal drilling and hydraulic fracturing. In the beginning of production, free gas was rapidly recovered through an artificial fracture system, and then, desorbed gas followed at the final stage due to a decrease of reservoir pressure. This desorbed gas volume commonly occupies 10 to 40% of total gas production in shale gas play although it shows wide variety in cumulative gas volume and production time. The largest gas production in Canada is recovered from either tight sandstone or shale reservoirs. The Montney play in Western Canadian Sedimentary Basin (WCSB) has produced up to 80% of Canadian natural gas production. The desorbed gas production from this play has been reported up to 10% of total produced gas. The distribution and productivity of the desorbed gas have not been fully studied. Therefore, we focus to understand the distribution of the desorbed gas volume of eastern, middle, and northwestern areas in the Montney play. The desorbed gas volume within these areas was estimated from the relationship among canister, illite, and shale volumes in core samples and well logs. The average shale volume fraction in eastern area is 0.38 v / v , the average illite mineral volume fraction is 0.25 v / v , and the average desorbed gas volume refers to 8.52 scf/ton. In middle area, calculated volume represents 0.34 v / v , 0.216 v / v , and 8.15 scf/ton as listed above. The northwestern area also shows 0.65 v / v , 0.4 v / v , and 9.78 scf/ton, respectively. 3D models of each area indicated relatively rich and lack parts of desorbed gas volume. These estimated desorbed gas volume and gas production history were compared in order to understand when and how the desorbed gas would affect to gas production. It shows strong positive relationship, gradually increasing correlation to the later stage (from 24-44 months to 36-44 months) of gas production in the entire areas. This result implies that the estimation of later stage gas productivity is able from the estimated volume of desorbed gas, and also, the total gas production can be forecast in shale gas reservoir. Northwestern area in Montney play preserves relatively abundant desorbed gas volume, which will be dominant after 24 months of production. [ABSTRACT FROM AUTHOR]

The Late Devonian Besa River Formation is an organic-rich shale sequence in Liard Basin, northeastern British Columbia, Canada, with significant natural gas reserves. High-resolution elemental geochemistry of three long continuous cored intervals of the Besa River Formation was used to better understand the paleodepositional environment of organic-rich intervals in this thick marine shale. The studied core intervals were divided into five chemostratigraphic units based on organic and inorganic geochemical proxies. The highest total organic carbon (TOC) content (up to 13 wt.%) was identified in the upper part of the Patry member (Unit III) within the Liard Basin. During the deposition of Unit III, low clastic influx and euxinic bottom conditions mostly contributed to the high accumulation of organic carbon. Moreover, a high productivity and organic influx may have increased organic-rich basinal sediments, which further depleted the seawater column oxygen content in the presence of a large amount of organic matter. This took place within the oxygen-deficient bottom water from the Patry–Exshaw stratigraphic units. This high TOC interval was most likely deposited through abundant biogenic silica production by radiolarians, thereby utilizing the supply of nutrients from the upwelling. Sea level change was also an important factor that controlled organic matter accumulation in the Besa River Formation. The transgression in sea level changed the residence time of the organic matter in oxic zones within the water column, which limited its supply in deeper water; this decreased the TOC content in Unit IV. Before the deposition, silica production collapsed and was replaced by terrestrial sedimentation of clay minerals in the upper part of the Exshaw member, which caused organic matter dilutions in Unit V (under 5 wt.%). These results provide new insights into the effects of relative sea level changes on redox conditions, productivity, and detrital flux, which are related to organic matter enrichment patterns and their geographic variations. Unit III is characterized by an organic-rich interval as well as an abundance of biogenic silica that is closely related to fracturing. Thus, Unit III is expected to have the highest shale gas potential in the Devonian Besa River Formation. The high-resolution geochemical data integrated with well log and/or seismic data can be used to determine the distribution of the perspective interval for shale gas production in the Liard Basin. [ABSTRACT FROM AUTHOR]

This study focuses on desorbed gas volume estimation of the Montney Formation, Deep Basin, Canada. Because desorbed gas has significantly contributed to shale gas production, and ranges from 10% to 40% of the total production, an accurate desorbed gas volume estimation is important to evaluate and develop shale gas fields. The desorbed gas volume was calculated using two methods – the adsorption capacity (cf. Langmuir test) and canister volume measurements. Both methods were originally adopted from coal-bed-methane production and applied to shale gas reservoirs after the commercial production of shale gas. The Montney Formation contains less than 2% of total organic carbon, with an average of 0.7% primarily derived from the secondary cracking of migrated oil. This suggests the absence of the adsorption capacity and organoporosity of organic matter. Conversely, the Montney Formation contains more than 20% of clay minerals, and the canister volume test shows that the formation has 22 scf/ton of desorbed gas volume. Based on a mineralogical analysis, this desorbed gas is primarily adsorbed to clay minerals, such as illite, mica, smectite, kaolinite, among others. Because illite is the major component among the clay minerals and has a positive correlation to the canister volume, the illite volume was calculated from conventional well logs to estimate exact desorbed gas volume. We assumed that the clay volume primarily consisted simply of illite and the rest clay minerals and figure out the volume of illite by using neutron-density cross plot. The calculated illite volume has a positive correlation to the canister volume, which can be used to infer the desorbed gas volume of the Montney Formation. The estimated desorbed gas volume in the Montney Formation ranges from 19 to 30 scf/ton, which is lower than the free-gas volume. [ABSTRACT FROM AUTHOR]

Evaluating the adsorbed gas content in shale gas reservoirs is crucial for calculating total gas production potential of the reservoir. In this study, we analyzed the chemical composition and total organic carbon content of the black shale samples collected from different depths in the Liard Basin, Canada. The samples were identified as siliceous-rich shale samples; specifically, organic-rich black shale with a total organic carbon content of more than approximately 3%. We also analyzed the adsorbed methane gas content and the pore structure of the shale samples. We conducted a mechanistic study to identify the important factors affecting the amount of adsorbed CH 4 gas in the shale by evaluating the correlation between the adsorbed CH 4 gas content and quantitative pore structure results (i.e., specific surface area and micropore, mesopore, and macropore volume). The results indicate that the micropore volume can be effectively used to predict the content of adsorbed CH 4 gas in organic-rich black shale samples from the Liard Basin. [Display omitted] • Black shales were collected from the Laird Basin natural gas deposit in Canada. • Organic-rich black shales exhibited similar chemical compositions and TOC contents. • The correlation between shale pore structure and adsorbed CH 4 content was analyzed. • Micropore volumes were relatively well correlated with adsorbed CH 4 contents. • Small pores are significant as an important factor in predicting the adsorbed CH 4 content in shale. [ABSTRACT FROM AUTHOR]

The shale gas potential of a Late Devonian shale sequence in the Liard Basin, an important unconventional gas reservoir, was assessed using the retained gas volume based on gas generation potential and hydrocarbon expulsion efficiency. Evaluation of gas generation potential in the Liard Basin depends on understanding the characteristics of the original organic matter. However, due to high thermal maturity, it is difficult to estimate the characteristics of the original organic matter from conventional methods. The type of original organic matter was inferred from inorganic geochemical proxies that reflect productivity (Ex-SiO 2), terrestrial sediment input (Al 2 O 3 , Zr), and oxygen level in the bottom water (Mo EF and U EF). By considering the present day TOC (TOCpd) and the type of original organic matter, the original hydrogen index (HIo) and original total organic carbon (TOCo) values were calculated to range from 200 to 700 mgHC/gTOC and from 0.3 to 29.0 wt%, respectively. The measured TOCpd values were similar to the TOCpd calculated from the 1-D petroleum system model using the original organic matter characteristics inferred in this study, which indicates that the TOCo values are reliable. Based on the evolution of the TOCo during its thermal history, the total gas generation potential in the Late Devonian shale sequence (550 ft) of the Liard Basin is estimated to be 1690 bcf/section. Considering the total gas in place (496 bcf/section), based on the estimated ultimate recovery, approximately 70% of the hydrocarbon might have been expelled. The retained gas volume fluctuates vertically in the Late Devonian shale sequence. The retained gas volume is higher in the 3946–3993 m (136 bcf/section) and 3993–4047 m (281 bcf/section) intervals than other intervals (less than 60 bcf/section), indicating that these intervals are the most prospective for shale gas exploration and development in the studied well. • The Late Devonian shale sequence in the Liard Basin is important unconventional gas reservoir. • Gas generation potential was estimated from TOCo inferred from inorganic geochemical data. • Based on the estimated ultimate recovery, 30% of gas generation potential is retained. • The estimated retained gas volume shows vertical change though the Late Devonian shale sequence. [ABSTRACT FROM AUTHOR]