ABSTRACT Exploration for hydrocarbons in Mississippian strata in Kansas and Oklahoma began in the 1900s. Early production came from open-hole completions in vertical wellbores at the apex of structural and stratigraphic traps. In the mid-20th century, cased-hole completions and hydraulic fracture stimulation allowed development of lower permeability zones. Recently operators began to explore and develop transition zones and low-permeability facies with horizontal drilling. The petroleum system that includes these accumulations consists of two hydrocarbon kitchens in the Arkoma and Anadarko basins, which have been generating oil and gas from the Woodford Shale since the beginning of the Pennsylvanian. Hydrocarbons charged out of the basins and along the fractured terrain of the Cherokee platform into reservoirs from Kinderhookian to Chesterian age across the carbonate facies belt. The distribution of these reservoirs, including limestones, dolomites, and cherts, along with structural configuration, governs the relative abundance and location of oil, gas, and water in each trap. The past decade saw over four thousand laterals targeting Mississippian reservoirs, including shales in unconventional traps, and the greatest rise in oil production in the region since the 1920s. High associated water volumes have created escalating operational costs and are correlative with earthquake activity.

ABSTRACT Late diagenesis records a common history of fluid flow in sub-Permian strata in the midcontinent, where fluid inclusion Th are higher than burial temperatures and Tm ice show evolving salinity. Most negative δ 18 O dolomite and highest Th are at the top of the Mississippian. Fluid inclusion and geochemical data point to advective fluid flow out of basins utilizing Cambrian–Ordovician–Mississippian strata as an aquifer for hydrothermal fluids. The Pennsylvanian was a leaky confining unit. This system evolved from: Stage 1 Pennsylvanian–early Permian pulsed hydrothermal migration of connate brine and gas; between Stages 1 and 2, low-temperature Permian brine reflux; Stage 2 mixing between high-temperature and low-temperature brines during the Permian; and Stage 3 large-scale migration of hydrothermal brines and oil later during the Permian or after. Stages 1–3 were the most important late processes affecting Mississippian reservoirs, and record an inverted thermal structure with most impact of hot fluids at the top of the Mississippian. Stage 4 shows radiogenic 87 Sr/ 86 Sr in calcite, supporting a transition to localized fault pumping from basement, likely driven by Laramide fault reactivation. Stage 5 is the current system, with Ozark and Front Range uplift-driven fluid flow and potential for small-scale sporadic fault pumping.

Abstract The localization and heterogeneity of carbonate oil and gas reservoirs are commonly controlled by extensive diagenetic alteration. Mississippian (Osagean–Meramecian) strata in SE Kansas are investigated to determine structural, relative sea-level, diagenetic and depositional controls on stratigraphy, lithofacies distribution and reservoir character. This project shows how karst horizons and fractured zones can provide preferred conduits for hydrothermal porosity enhancement. Thus, enhanced porosity in karst horizons may have a late origin, with chemically aggressive hydrothermal fluids following preferred pathways of fluid flow. Lithofacies include echinoderm-rich bioclastic wacke–packstone, sponge-spicule-rich packstone, dolomitic bioclastic wackestone, argillaceous dolomite, tripolitic chert and chert breccia. Four cores are used to construct a 10 mile-long SW–NE-trending cross-section, showing three genetic units deposited on a mostly south-facing distally steepened ramp, with periods of upwelling. Paragenesis reveals that early and late dissolution enhances porosity in chert and carbonate facies. Fluid inclusion microthermometry from megaquartz and baroque dolomite reveals variable but increasing homogenization temperatures (70–160 °C) and increasing salinity through time. The best reservoirs may be controlled by depositional setting that led to large amounts of chert, alteration associated with subaerial exposure, and a hydrological and structural setting that led to enhanced hydrothermal fluid flow for later dissolution.

Abstract During the Carboniferous a carbonate shelf covered areas of the central U.S, including Kansas, with the outer shelf and shelf margin intermittently extending through southern Kansas and northern Oklahoma. The regional setting resulted in deposition of relatively laterally continuous carbonate and siliciclastic facies belts. Areally sparse subsurface well data and surface exposures have led to the interpretation that most structures on the craton are simply shallow draped anticlines and associated synclines primarily reflecting general aspects of regional structure and depositional and erosional heterogeneity. Models that assume a broad continuous shelf relegate local structure to being minor or non-existent. However, our recent examination of subsurface data, 3-D seismic, and rock properties for oil fields from the Middle Mississippian shelf margin, Middle Pennsylvanian mid-shelf, and Late Pennsylvanian lower shelf indicate that regional- and kilometer-scale structures (e.g., faults, fractures, lineaments) segmented the shelf and shelf-margin areas in Kansas, primarily along Precambrian structures that were reactivated throughout the Phanerozoic. Movement on faults resulted in segmentation expressed as rhombic-shaped structural blocks (1-10s km) with subtle variations in relief (generally meter to ~ 70 m) and slope (near zero to upwards of 2-3 m/km). Regional, down-to-basin block faulting produced linear shelf edges and segmentation of the ramp and shelf profile repeatedly during the Carboniferous. The association of stratal packages and rock properties with structural elements argues that structure exerted continued, but episodic, influence and affected sediment accommodation, depositional patterns, paleotopography, weathering intensity, diagenesis, and later fluid movement, including hydrocarbon emplacement. Results from our study of the “stable” shelf carbonates of the Midcontinent indicate that tectonic events may have had far-reaching effects and caused structural deformation in the interiors of cratons. Sedimentologic and stratigraphic analyses in such settings can benefit by evaluating the possible influence of subtle faulting and fault reactivation on depositional and diagenetic patterns that can significantly influence rock properties and reservoir development.

Abstract Although the general aspects of oolitic depositional systems are well-documented, their landscape-scale patterns (geobodies) are not well enough understood to offer quantitative, predictive insights for reservoir characterization. To begin to fill this basic gap in understanding, this study describes the morphology, hydrodynamics, and process sedimentology of several modern tidally dominated Bahamian ooid shoal complexes and compares the patterns with patterns in Kansas Pennsylvanian analogs. A companion paper explores linkages further, documenting petrophysical, geophysical, and production characteristics of these Pennsylvanian oolitic reservoirs. Integrating remote sensing imagery with quantitative bathymetric, fluid flow, and granulometric data in a GIS , we document geomorphic and sedimentologic patterns and processes in several active tidally-dominated shoals. Results reveal that parabolic bars form a common morphologic motif, although there is considerable variation on that general theme. Different processes can lead to varying depositional geometries and sedimentologic patterns. Nonetheless, the landscape-scale configuration of bars and superimposed sand waves is linked closely to patterns of tidal flows. Bars are not homogenous bodies, however, and granulometric parameters such as sorting and mud percentage vary systematically and predictably within the hydrogeomorphic framework. Through exploring modern oolitic shoals, this study provides new insights on details of their morphology and dynamics as well as links between geomorphic framework and grain size and sorting; some patterns are similar to those within geobodies in Pennsylvanian reservoir analogs. These insights provide quantitative predictive information on facies geometries, on grain characteristics, and depositional porosity in analogous ancient ooid shoals.

Abstract Data used to build reservoir models come from different scales, such as core, log, well tests, pressure and production profiles, and seismic; thus they carry the inherent need for calibration to a common scale. Unfortunately, as no accepted procedure is available to solve this calibration problem, doubts remain about the representativeness of the data that are often used to describe a reservoir model. In the absence of a standardized upscaling method, a series of procedural steps, described in this paper, can be employed on data gathered from different sources to test and build a coherent reservoir model. Each step in this procedure is a part of an iteration loop that checks for consistency and coherency among the available data. In case of a mismatch, the process encourages the user to go back to the previous step or steps and revise one or more of the relevant assumptions. The method outlined in this paper, the Super Pickett crossplot, integrates data from different sources such as cores, well logs, and well performance to build a volumetric geomodel that is then used to validate the mass balance calculation. The strength of this method lies in the fact that the major elements (the Super Pickett plot, the volumetric and material balance calculations) of the process can be performed in a spreadsheet environment, making it both cost effective, versatile, and portable. This exercise enables the building of an internally consistent geo-engineering model representing the reservoir. Such a model can be used effectively as the basis for reservoir simulation studies.

Abstract Numerical Experiments in Stratigraphy: Recent Advances in Stratigraphic and Sedimentologic Computer Simulations - This volume presents the results derived from a three-day workshop held at the University of Kansas, Lawrence, Kansas, from May 15 through May 17, 1996. The objectives of the workshop were to document, characterize, demonstrate, and compare different computing procedures that have been utilized in simulating stratigraphic sequences. Both inverse and forward simulation modeling procedures are represented. The results of the workshop and the papers assembled here include: (1) an enhanced understanding of similarities and differences between models and modeling philosophies, (2) increased communication among modeling groups and geoscientists, (3) critical evaluation of applications and assessment of how models have been utilized, and (4) improvements and refinements in techniques for generating and describing model input and output.

Abstract Computer stratigraphic simulation models provide a quantitative means to evaluate and understand complex interactions of sedimentary depositional systems. People in the geosciences are quickly advancing in their ability to acquire and interpret large data sets, resulting in major advances in understanding earth systems. Simulation is a natural outcome of these advances, as is the need to integrate and process this information. This volume provides a collection of 26 papers that describe and illustrate the application of some of the latest approaches to stratigraphic-sedimentologic modeling. This paper serves as an overview of these papers, classifies modeling, reviews current issues of modeling, and evaluates possible future modeling directions and opportunities. We have recognized several different approaches to modeling and present a rational classification for these model types, illustrated here and in the volume by diverse examples. Despite varying philosophies and methodologies of their creators, most models consist of three essential components: (1) input, (2) engine, and (3) output. Our results suggest that models have a sound observational basis (input) and logical foundation (engine), both of which use ever-improving quantitative knowledge of geologic systems. Roles of modeling include: (1) encouraging accuracy and precision in data collection and process interpretation (Slingerland et al., 1994); (2) providing a means to quantitatively test interpretations of the roles of various driving mechanisms to produce sedimentary packages; (3) predicting or extrapolating results into areas of limited control; (4) affording mechanisms for enhanced multidisciplinary integration and communication; (5) gaining new insights to offer nonintuitive results regarding the interaction of parameters; and (6) helping focus future studies to resolve specific problems. The future of modeling is dependent upon fully using improved computational methods and machines, refining quantitative geologic observations and interpretations, and developing rigorous, quantitative approaches to testing, calibrating, verifying, and comparing models.

Abstract This document provides a record of the discussions conducted by the Geological Observations and Parameterizations (GOP) group, which sought to define and address important questions concerning the parameterization of geological observations for use in stratigraphic simulations. The discussions of this group, which was intermittently composed of a combination of model-building, model-using, and nonmodeling geologists, centered on the type and quality of common geological observations, the scales of these observations, and the accurate representation of observations in geological models. The underlying concerns were whether all relevant processes are accounted for in existing models, and whether modelers are using the available models correctly, inputting realistic values and ranges of values at the proper scales. Our fundamental questions include the following: How closely is the variability of geological observations reflected in stratigraphic models? If such variability is well represented within a model, how well does a simulation predict away from points of control? If it does not do this well, is it the input parameters that are not reflecting the natural variability of the system, or does the fault lie with the model itself? If the model itself is the limiting factor, how well do we really need to constrain input parameters? How well do we understand the nonlinear characteristics of natural systems, and how are these characteristics introduced into models?

Abstract The purpose of inversion is to determine the limits of prediction for model testing and risk assessment given the state of our knowledge (knowledge in the form of model assumptions, accuracy, and precision, and in the form of data distributions, types, accuracy, and precision). Inversion is a tool for making decisions. Should we do inversion? Are current models and assumptions sufficient to solve posed problems, or do we need to develop a new model? Are currently available data sufficient, or do we need to collect more or different data? And how do we decide? The above plethora of queries arose during the committee discussions as general concerns, as did a large number of more specific questions. We record in Appendix 1 all of the questions that arose because the committee deliberations did not allow time for complete resolution of all issues. We recorded all of the questions in the hope that some, at least, will be answered in the future. The question list of Appendix 1 is far too long to address in detail, so the committee focused on attempts to provide some pragmatic rules of operation to guide the actual technical use of inverse modeling. Although the rules set down in this paper are not a universal panacea, they do represent considerable practical experience with inverse models. As such, they are useful guides as one struggles to determine the strengths and weaknesses of inverse modeling.

Abstract A procedure determines the relative importance of uncertainties in input information and in multiple parameter estimation to all outputs from two-dimensional basin modeling codes. The procedure does not rely on Monte Carlo methods, but on some simple properties of the cumulative probability distribution of output variations related to uncertainties. As a consequence, only a couple of computer trials are needed to evaluate the relationship of the variability of outputs to input uncertainties. The procedure is applied to a two-dimensional cross section with evolution of the section with time. Attention is focused first on mainly geologic input uncertainties, and then on uncertainties of thermal factors and of hydrocarbon kinetic factors. Each group is initially taken separately, and then all three groups of uncertainties are combined and used simultaneously. The influence of each group of uncertainties on a suite of different outputs from the basin model is explored at different times across the evolving section. At each lime step, the relative sensitivity is examined of the uncertainty in a specified output to each group of input uncertainties, as is the relative importance of the uncertainty in a specified input to the suite of all outputs at each time step. In addition, the global relative importance of input uncertainties to output variabilities is considered, thereby providing a measure of output uncertainty effects, no matter where and when they occur, as a consequence of input uncertainties. This work enables one to assess which inputs need to be more tightly constrained, and also to determine by how large a factor they need to be better constrained if the uncertainties on a suite of specified outputs are to remain within given tolerance limits. The advantage to this rapid procedure is that one can focus more quickly on those factors of dominance in controlling, say overpressure development or hydrocarbon charge in a basin, without having to spend an inordinate amount of time, effort, or financial or staff resources on providing narrower limits of uncertainty to those input factors that provide but little change in output uncertainties.

Abstract Stratigraphic inversion is a quantitative technique that extracts values of process parameters, such as tectonic movement, lithosphere strength, sea level change, sediment supply, and basin topography, from stratigraphic data. A stratigraphic inverse model contains (1) a forward model that simulates stratigraphy through the operation of a set of input process parameters and algorithms that describe the behavior of the stratigraphic process-response system; (2) a set of observed data that are comparable in type and form to forward model predictions; and (3) a set of equations and algorithms that compare the values of forward model predictions with observations, and simultaneously adjust values of all forward model parameters to create a better match between predictions and observations. The inverse model iteratively reduces differences between forward model predictions and observations until a best match is achieved. The model calculates the degree of accuracy and uncertainty of values of stratigraphic predictions. Constructing an inverse model requires the following steps: (1) selecting a stratigraphic forward model; (2) designing simple mathematical functions that most accurately describe the real stratigraphic processes that operated in a basin and that make inversion computationally possible; (3) measuring data that correspond in type to the output of that forward model and transcribing those data into the form of a numerical vector; (4) selecting an appropriate parameter optimization algorithm; and (5) building a stratigraphic inverse model that connects components of steps 1–4. One purpose of stratigraphic inverse modeling is predicting stratigraphic attributes (e.g., facies, geometry, distribution, volume) with calculated estimates of accuracy and uncertainty. Once the range of parameter values is calculated by the inverse model, a population of forward models may be run that should contain the true stratigraphy. The population of forward models is used to predict the geographic and stratigraphic positions and extent of potential reservoir and source and seal rocks. We show an example of accurate stratigraphic predictions using inverse modeling of the Mesa Verde Group, San Juan basin, Colorado and New Mexico, United States.

Abstract Stratigraphic modeling involves a multidimensional parameter fitting problem where a large number of free model parameters have to be adjusted for the model to match observational data. This task can be viewed as an optimization problem, which here is addressed using a genetic algorithm. The iterative trying-and-checking process, usually done manually, is thereby automated. We apply this method for the automatic construction of sea level and subsidence curves for two simple toy models. We also address the problem of distinguishing the sea level variations vs. subsidence variations, and we give an example of a simulation involving carbonates from Mallorca, Spain.

Abstract Numerical-statistical algorithms are used to model end-member grain-size distributions of pelagic and hemipelagic siliciclastic sediments of the Arabian Sea. The grain-size distributions of sediments from the Oman continental slope, the Owen Ridge, the Pakistan continental slope, and the Indus Fan can be adequately described as mixtures of three end members. The spatial variation in relative contribution of the end members is interpreted in terms of transport processes and provenance. In the western Arabian Sea, deposition is dominated by two end members that represent "proximal" and "distal" eolian dust. A third end member, which dominates the deposits of the middle Indus Fan, represents fluvial mud deposited from low-density turbidity currents (lutite flows). At any given location, the temporal changes in the relative contribution of the end members can be interpreted in terms of climate change. The ratio of contributions of the two eolian end members (i.e., the grain-size distribution of the eolian dust) on the Owen Ridge (NIOP492) reflects the strength of the summer monsoon. Deposition on the upper Indus Fan (NIOP458) is dominated by "distal" eolian dust and fluvial mud. The ratio of contributions of eolian and fluvial sediment reflects continental aridity. The ratio of contributions of the two eolian end members (i.e., the grain-size distribution of the eolian dust) on the upper Indus Fan reflects the strength of the winter monsoon. Our reconstruction of the late Quaternary variations in Arabian Sea monsoon climate corresponds well with interpretations of the loess-paleosol sequences on the Chinese Loess Plateau. In both areas, the bulk of the annual precipitation is confined to the summer monsoon season. Intensification of the summer monsoon during interglacials, which has been identified as the principal control on pedogenesis on the Loess Plateau, also explains increased discharge of Indus River-derived muds to the northern Arabian Sea. Independent evidence for summer monsoon strength, provided by the eolian grain-size record of the western Arabian Sea, fully supports this conclusion. The strength of the summer monsoon thus provides an aridity forcing mechanism for both the Arabian Sea and the Loess Plateau. The grain size of the eolian dust in the northern Arabian Sea and on the Loess Plateau indicates intensified winter monsoons during glacials.