ABSTRACT Mississippian rocks in north-central Oklahoma were deposited on a ramp-shelf system that trended along an approximate northeast–southwest strike and that deepened to the southeast and southwest into the Arkoma and Anadarko basins. The system is bounded on the east by the Ozark uplift. Structure in this area is dominated by extensional and transverse faulting associated with the Transcontinental arch (Nemaha uplift). Shallower water (shelf) depositional settings dominate in the northern part of the study area and deepen toward the south into the Anadarko and Arkoma basins. Sedimentary rocks on the carbonate ramp are dominated by cyclic, partially dolomitized, argillaceous mudstones interbedded with fine-grained wackestones to grainstones. Intergrain pore space is filled by bladed, isopachous, and syntaxial marine calcite cements followed by blocky calcite cements. Limestone is commonly replaced by chert with intergrain open space filled by fine crystalline quartz (chert) cement. Late diagenetic fracture, breccia, and vug (FBV) porosity are filled by calcite and less commonly, by quartz cement that displays a coarse, blocky habit. Carbon and oxygen isotope values for limestones and replacive dolomite are consistent with precipitation from Mississippian seawater and mixed seawater–meteoric water; values for FBV-filling calcite cements indicate precipitation from evolved basinal waters. The 87 Sr/ 86 Sr values of calcite micrite, replacement dolomite, and fracture-filling calcite range from 0.7077 to 0.7112. The lower values are consistent with equilibration with Mississippian seawater through most of the study area. More radiogenic 87 Sr/ 86 Sr values for fracture-filling calcite cements in the northeast part of the study area indicate interaction with continental basement rocks or siliciclastic rocks derived from continental basement. Two-phase (liquid plus vapor) aqueous and petroleum inclusions were observed in FBV-filling calcite and quartz cements. The aqueous inclusions have homogenization temperatures of 48°C to 156°C and salinities ranging from 0 to 25 equivalent weight % NaCl equivalent, and reflect the presence of distinct dilute and saline fluid end-members. Calculated equilibrium δ 18 O water values (VSMOW) for fluids that precipitated fracture-filling calcite cements are variable, ranging from –0.3 to +14.5‰ and do not reflect a single end-member water. Early diagenesis was dominated by seawater-involved cementation, with modification by meteoric water during sea-level low-stands. FBV-filling calcite and quartz represent a later stage of diagenesis associated with petroleum generation and migration. Formation of fractures in the Mississippian section in north-central Oklahoma likely is related to fault movement along the Nemaha ridge instigated by Ouachita tectonism during the Pennsylvanian and extending into the Permian. This timing corresponds with regional flow of saline basinal fluids associated with the orogenic activity. These fluids ascended along faults and contributed to precipitation of FBV-filling cements. Calculated δ 18 O water values for calcite cement in some areas of north-central Oklahoma suggest that cement-depositing fluids approached isotopic equilibrium with the host carbonate rocks. In other areas, however, cement-depositing fluids have oxygen isotope signatures that reflect nonresident fluids whose flow was restricted to fault and fracture pathways, which did not permit isotopic equilibration with the host limestone. In particular, fracture-filling calcite veins from Osage County, with high 87 Sr/ 86 Sr (>0.710) and low δ 13 C values (–2.3‰ to –4.1‰), reflect fluids that retained isotopic characteristics that were derived through interaction with subjacent shale source rocks.

Abstract Remaining resources in mature basins, such as the southern North Sea (SNS), are often associated with complex or unconventional reservoirs. Unlocking the value of these resources requires non-conventional approaches to the description and the development of the reservoir. Originally developed as a conventional reservoir, the Hoton field in the SNS delivers economic rates from a tight and fractured Lower Leman Sandstone. The northern part of the field is developed via a trilateral producer, but the field’s southern half, which appraisal drilling showed to consist of poor-quality rock, was deemed too tight for development. Roughly 40% of the field’s resource of dry gas lies within the southern part of the field with no plans to develop it. A reassessment of the southern area’s potential using geomechanical tools suggests that it also could deliver commercial well rates, similar to those experienced in the northern area of the field. A combination of structural restorations using Dynel2D and forward deformation modelling using Poly3D produces palaeostress predictions, which are used to constrain a discrete fracture network model. From these, well profiles are generated that suggest that a development well into the southern part of the field can recover about half of the currently stranded resource.