Rock mechanical properties of immature, organic-rich source rocks and their relationships to rock composition and lithofacies

Mechanical properties of layered rocks are critical in ensuring wellbore integrity and predicting natural fracture occurrence for successful reservoir development, particularly in unconventional reservoirs for which fractures provide the main pathway for hydrocarbon flow. We examine rock mechanical properties of exceptionally organic-rich, immature source rocks from Jordan, and understand their relationships with rock mineral composition and lithofacies variations. Four depositional microfacies were identified: organic-rich mudstone, organic-rich wackestone, silica-rich packstone and fine-grained organic-rich wackestone. The four types exhibit various mineralogical compositions, dominated by carbonates, biogenic quartz and apatite. Leeb hardness ranges between 288 and 654, with the highest average values occurring in silica-rich packstone and organic-rich mudstone. The highest uniaxial compressive strength (derived from the intrinsic specific energy measured using an Epslog Wombat scratch device), and compressional- and shear-wave velocities were measured in organic-rich mudstones (140 MPa, 3368 m s−1 and 1702 m s−1, respectively). Porosity shows higher average values in organic-rich wackestones and fine-grained organic-rich wackestones (33–35%). Silica-rich packstone and organic-rich mudstone have brittle properties, while organic-rich wackestone and fine-grained organic-rich wackestone are ductile. High silica contents are correlated positively with brittleness. A strong hardness–brittleness correlation suggests that Leeb hardness is a useful proxy for brittleness. Our study allows a better understanding of the relationships between lithofacies, organic content and rock mechanical properties, with implications for fracking design to well completion and hydrocarbon production. Further work involving systematic sampling and a more rigorous study is still required to better understand the spatial distribution of target lithologies and their mechanical properties

Influence of organics and gas mixing on hydrogen/brine and methane/brine wettability using Jordanian oil shale rocks: Implications for hydrogen geological storage

The substitution of fossil fuel with clean hydrogen (H2) has been identified as a promising route to achieve net zero carbon emissions by this century. However, enough H2 must be stored underground at an industrial scale to achieve this objective due to the low volumetric energy density of H2. In underground H2 storage, cushion gases, such as methane (CH4), are required to maintain a safe operational formation pressure during the withdrawal or injection of H2. The wetting characteristics of geological formations in the presence of H2, cushion gas, and the resultant gas mixture in the mixing zone between them are essential for determining storage capacities. Therefore, the present work measured the contact angles of four Jordanian oil shale rocks with H2, CH4, and H2-CH4/brine mixture systems and their interfacial tension (IFT) in geological storage (geo-storage) conditions (pressures of 0.1 to 1600 psi and temperature at 323 K) to evaluate the residual and structural trapping potential and efficiency of CH4 as a cushion gas. Various analytical methods were employed to comprehend the bulk mineralogy, elemental composition, topographic characterization, functional groups, and surface properties of the Jordanian oil shale rocks. The total organic carbon (TOC) effect on wettability was demonstrated and compared with previous studies. The Jordanian oil shale samples with high to ultrahigh TOC of 13 % to 18 % exhibited high brine advancing/receding contact angles. The rock samples became hydrophobic at the highest experimental pressure and temperature conditions (1600 psi and 323 K). The rock/CH4/brine contact angles were higher than the rock/H2/brine contact angles, and the H2-CH4/brine mixture contact angles remained in between those for pure gases. Moreover, the IFT displayed the inverse trend, where the H2/brine IFT measured higher than the CH4/brine IFT. The results suggest that the H2 geo-storage in the tested organic-rich source rocks could be favorable when CH4 is used as a cushion gas, consistent with previous studies using synthetically acid-aged shale samples. For the first time, the present work used organic-rich rocks from Jordanian oil shale to present a more realistic situation and evaluate the influence of missing organic material and gas on the H2/brine/rock wettability during H2 geo-storage

Diagenesis, compaction strain and deformation associated with chert and carbonate concretions in organic‐rich marl and phosphorite; Upper Cretaceous to Eocene, Jordan

This paper presents an integrated petrographic–geochemical–geomechanical study of the growth mechanisms of carbonate and chert concretions observed at outcrop and core from the Upper Cretaceous to Eocene organic-rich carbonate mudrocks, central Jordan. It provides evidence for displacive and replacive concretion growth from the analysis of primary lithological characteristics, compaction strain and deformation structures associated with concretion growth. Concretions were analysed to determine the primary lithological controls on their development and the measurement of strain in the host rock to develop a method for constraining the growth mode and their paragenesis. Concretions exhibit either a replacive or displacive growth mode largely dependent on the original host lithology. Displacive concretions exhibit irregular shapes and semi-fibrous internal structures in contrast to regular shapes and microcrystalline textures observed for replacive concretions. Cement fraction is high in both carbonate concretion types, indicating early formation in high-porosity sediments at shallow burial depths. The strain field around displacive concretions is vertically asymmetrical. Conversely, it is symmetrical with uniform differential compaction for the replacive concretions. Evidence for displacive growth comes from triangular areas of chert at the lateral margins of some carbonate concretions, interpreted as areas of reduced strain. Another indicator is the forced asymmetrical folding of heterolithic host rocks around displacive concretions, with displacive carbonate units separated by trace laminae of the original (chert) beds. Enveloping chert beds exhibit early-formed radial silica fractures with increased aperture size in the areas of maximum curvature. Carbon isotopic signatures of carbonate concretions show a strong correlation between concretion centres and host rock, suggesting a relatively shallow depth (first few tens of metres) of initial growth. Carbonate concretions are interpreted to have formed at shallow depths in the presence of alkaline pore waters rich in dissolved organic carbon in the presence of Mg2+ ions, available organic matter and redox-sensitive metals such as U and Mo. A paragenetic history for the different concretion types is presented

Genesis and role of bitumen in fracture development during early catagenesis

Bitumen-bearing fractures and vugs were investigated in the highly organic-rich Jordan Oil Shale (JOS) of Late Cretaceous to Eocene age, which has potential as a highly fractured, unconventional hydrocarbon play. Bitumen is present as macroscopically visible deposits and as inclusions in the cement of abundant natural fractures and adjacent vugs. The frequency of bitumen occurrence in fractures closely correlates with Total Organic Carbon (TOC) and burial depth. Petrographic and organic-geochemical analyses on bitumen samples extracted from fractures and their host rock matrix show that the fracture-filling bitumen comprises indigenous low maturity hydrocarbons derived from the surrounding organic-rich Oil Shale and has not migrated from a deeper source. Maturity indicators imply that the oil shale is in the pre-oil generation stage of early catagenesis throughout the investigated area, but with a regional increase in thermal maturity from west to east as the result of greater maximum burial depth. Bitumen mobilization in the host rock was mainly controlled by vertical loading stress acting on the non-Newtonian bitumen phase in load bearing configurations in the organic-rich matrix. Bitumen fractures were developed by hydraulic fracturing as the result of fluid overpressure in the organic matter. Overpressured bitumen has acted as a fracture driver, generating bitumen veins in both the organic-rich mudstones and the adjacent chert and silicified intervals

Stress distribution around kerogen particles as a measure of the initiation of bitumen-filled microfractures in organic-rich source rocks

In this article, we present a method used to model the initiation of bitumen-filled microfractures in immature, organic-rich source rocks. The first part presents the method used to calculate the stress distribution around the kerogen particles. The second part explains the method used to calculate the pressure change as a function of the transformation ratio and the resulting overpressure. The effective principal stresses acting on the kerogen boundary were calculated. Kerogen geometries were determined using the measured aspect ratio of the kerogen traces obtained from the petrography observation. To estimate overpressure, the increase in pressure due to the transformation of kerogen to bitumen was calculated

Modelling the initiation of bitumen-filled microfractures in immature, organic-rich carbonate mudrocks: The Maastrichtian source rocks of Jordan

The initiation of bitumen-filled microfractures was analysed in the organic-rich Maastrichtian carbonate mudrocks of Jordan, which show great potential as source rocks and for a future unconventional hydrocarbon play. A modelling approach was performed to assess the possible scenarios causing horizontal small-scale (mm to cm in length) bitumen fractures (microfractures) at the immature stage. The aim was to back-calculate how much overpressure and bitumen generation was needed in the past to initiate horizontal microfracturing, comparing those simulated parameters with the actual generation potential from the source rock samples. The results show that the local overpressure resulting from the bitumen generation during early catagenesis was not high enough to initiate the microfracturing. We hypothesise that the increase of internal pressure was caused by the inability of the bitumen to be squeezed into the pore space during burial. The resulting overpressure induced a perturbation to the stable-state stress distribution around the kerogen boundary that eventually led to the initiation of horizontal microfractures along the tip of bitumen flakes. Subsequently, short-distance migration of bitumen and a significant decrease in pressure have prevailed in the study area. This proves that primary migration can occur long before the source rock reaches the oil or gas windows, at a comparatively shallow burial depth. This also indicates that the first framework pathways by the precursor horizontal microfractures may control the flow patterns of the hydrocarbons within source rocks. Understanding these factors is critical to predicting the impact of these microscale fractures on hydrocarbon expulsion and storage, and hence likely productivity of an analogous subsurface unconventional reservoir

Application of Combined Local and Global Optimization Algorithms in Joint Interpretation of Direct Current Resistivity and Seismic Refraction Data: A Case Study of Dammam Dome, Eastern Saudi Arabia

The main geological structures in the Dammam Dome are defined by integrating geophysical measurements and applying new methodological approaches. Dammam Dome is characterized by a well-developed fracture/joints system; thus, high complexity of the subsurface is expected. Direct Current Resistivity (DCR) and Seismic Refraction (SR) geophysical survey aimed to map the Dammam Dome’s near-surface features. The geophysical data were acquired along two profiles in the northern part of Dammam Dome. To maximize the results from conducting DCR and SR measurements over a complex area, a combined local and global optimization algorithm was used to obtain high-resolution near-surface images in resistivity and velocity models. The local optimization technique involves individual and joint inversion of the DCR and SR data incorporating appropriate regularization parameters, while the global optimization uses single and multi-objective genetic algorithms in model parameter estimation. The combined algorithm uses the output from the local optimization method to define a search space for the global optimization algorithm. The results show that the local optimization produces satisfactory inverted models, and that the global optimization algorithm improves the local optimization results. The joint inversion and processing of the acquired data identified two major faults and a deformed zone with an almost N–S direction that corresponds with an outcrop were mapped in profile one, while profile two shows similar anomalies in both the resistivity and velocity models with the main E–W direction. This study not only demonstrates the capability of using the combined local and global optimization multi-objectives techniques to estimate model parameters of large datasets (i.e., 2D DCR and SR data), but also provides high-resolution subsurface images that can be used to study structural features of the Dammam Dome

Global variation in postoperative mortality and complications after cancer surgery: a multicentre, prospective cohort study in 82 countries

© 2021 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY-NC-ND 4.0 licenseBackground: 80% of individuals with cancer will require a surgical procedure, yet little comparative data exist on early outcomes in low-income and middle-income countries (LMICs). We compared postoperative outcomes in breast, colorectal, and gastric cancer surgery in hospitals worldwide, focusing on the effect of disease stage and complications on postoperative mortality. Methods: This was a multicentre, international prospective cohort study of consecutive adult patients undergoing surgery for primary breast, colorectal, or gastric cancer requiring a skin incision done under general or neuraxial anaesthesia. The primary outcome was death or major complication within 30 days of surgery. Multilevel logistic regression determined relationships within three-level nested models of patients within hospitals and countries. Hospital-level infrastructure effects were explored with three-way mediation analyses. This study was registered with ClinicalTrials.gov, NCT03471494. Findings: Between April 1, 2018, and Jan 31, 2019, we enrolled 15 958 patients from 428 hospitals in 82 countries (high income 9106 patients, 31 countries; upper-middle income 2721 patients, 23 countries; or lower-middle income 4131 patients, 28 countries). Patients in LMICs presented with more advanced disease compared with patients in high-income countries. 30-day mortality was higher for gastric cancer in low-income or lower-middle-income countries (adjusted odds ratio 3·72, 95% CI 1·70–8·16) and for colorectal cancer in low-income or lower-middle-income countries (4·59, 2·39–8·80) and upper-middle-income countries (2·06, 1·11–3·83). No difference in 30-day mortality was seen in breast cancer. The proportion of patients who died after a major complication was greatest in low-income or lower-middle-income countries (6·15, 3·26–11·59) and upper-middle-income countries (3·89, 2·08–7·29). Postoperative death after complications was partly explained by patient factors (60%) and partly by hospital or country (40%). The absence of consistently available postoperative care facilities was associated with seven to 10 more deaths per 100 major complications in LMICs. Cancer stage alone explained little of the early variation in mortality or postoperative complications. Interpretation: Higher levels of mortality after cancer surgery in LMICs was not fully explained by later presentation of disease. The capacity to rescue patients from surgical complications is a tangible opportunity for meaningful intervention. Early death after cancer surgery might be reduced by policies focusing on strengthening perioperative care systems to detect and intervene in common complications. Funding: National Institute for Health Research Global Health Research Unit

Effects of hospital facilities on patient outcomes after cancer surgery: an international, prospective, observational study

© 2022 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY 4.0 licenseBackground: Early death after cancer surgery is higher in low-income and middle-income countries (LMICs) compared with in high-income countries, yet the impact of facility characteristics on early postoperative outcomes is unknown. The aim of this study was to examine the association between hospital infrastructure, resource availability, and processes on early outcomes after cancer surgery worldwide. Methods: A multimethods analysis was performed as part of the GlobalSurg 3 study—a multicentre, international, prospective cohort study of patients who had surgery for breast, colorectal, or gastric cancer. The primary outcomes were 30-day mortality and 30-day major complication rates. Potentially beneficial hospital facilities were identified by variable selection to select those associated with 30-day mortality. Adjusted outcomes were determined using generalised estimating equations to account for patient characteristics and country-income group, with population stratification by hospital. Findings: Between April 1, 2018, and April 23, 2019, facility-level data were collected for 9685 patients across 238 hospitals in 66 countries (91 hospitals in 20 high-income countries; 57 hospitals in 19 upper-middle-income countries; and 90 hospitals in 27 low-income to lower-middle-income countries). The availability of five hospital facilities was inversely associated with mortality: ultrasound, CT scanner, critical care unit, opioid analgesia, and oncologist. After adjustment for case-mix and country income group, hospitals with three or fewer of these facilities (62 hospitals, 1294 patients) had higher mortality compared with those with four or five (adjusted odds ratio [OR] 3·85 [95% CI 2·58–5·75]; p<0·0001), with excess mortality predominantly explained by a limited capacity to rescue following the development of major complications (63·0% vs 82·7%; OR 0·35 [0·23–0·53]; p<0·0001). Across LMICs, improvements in hospital facilities would prevent one to three deaths for every 100 patients undergoing surgery for cancer. Interpretation: Hospitals with higher levels of infrastructure and resources have better outcomes after cancer surgery, independent of country income. Without urgent strengthening of hospital infrastructure and resources, the reductions in cancer-associated mortality associated with improved access will not be realised. Funding: National Institute for Health and Care Research