John W. Harbaugh

Simulating clastic sedimentation

This book discusses a new computer methodology capable of simulating the erosion, transport and deposition of clastic sediment. It describes simulation models, a series of computer programs written in FORTRAN, representing geologic processes acting in three dimensions through geologic time. It also provides the theoretical and experimental foundations for the procedures used and a mathematical introduction to the components of the earths dynamic system involving running water.

Reassessment of the Rates at which Oil from Natural Sources Enters the Marine Environment

Abstract Previous estimates of the world-wide input of oil to the marine environment by natural seeps ranged from 0·2 to 6·0 million (metric) tonnes per year with a ‘best estimate’ of 0·6 million tonnes per year. Based on considerations of the availability of oil for seepage from the worlds known and assumed oil resources, we believe that the world-wide natural oil seepage over geological time should be revised to about 0·2 million tonnes per year with a range upward or downward of a factor of ten leading to estimates between 0·02 and 2 million tonnes per year. Our estimate of the amount of oil eroding from the land and being transported to the oceans is about 0·05 million tonnes per year with an order of magnitude uncertainty. Therefore, while the uncertainties are large, we estimate that the total amount of oil entering the marine environment by natural, geological processes, is about 0·25 million tonnes per year, and the estimate may range from about 0·25 to 2·5 million tonnes per year.

DEPOSIM: A Macintosh computer model for two-dimensional simulation of transport, deposition, erosion, and compaction of clastic sediments

Abstract DEPOSIM is a dynamic deterministic two-dimensional simulation model implemented on an Apple Macintosh 512 kbyte computer, that represents the interaction between transport, deposition, erosion, and compaction of clastic sediments. Sediment particles, transported within a fluid medium, are represented in a cross section through a sedimentary basin. The cross section is separated into discrete columns. The behavior of particles depends on velocity of the fluid in each column of the cross section, as well as on the basins configuration, particularly on the steepness of slope of the seafloor. Fluid velocity depends on the velocity of the fluid newly supplied to the basin in each time increment, and on the depth of water in each column. Input parameters were taken from actual sedimentological and oceanographic literature that is concerned with clastic sediments. DEPOSIM takes into account up to three different particle sizes. Graphics subroutines display the cross sections and frequency distributions of particle sizes on a column-by-column basis. Resolution of cross sections on the computer screen or in hardcopies can be improved by using an “enlarging” subroutine. Preparation of data input for the model is interactive completely and uses mouse and window techiques. DEPOSIM is well suited for computer-aided instruction.

Granitic rocks of Yosemite Valley and an ideal granite model

Stochastic models of an ideal, a “degenerate” ideal granite, and two models for metasomatically transformed granites have been compared with sequences of grains observed in granitic rocks of Yosemite Valley, California. Properties of the granitic rocks appear to show a relationship to the topography and stream directions, and may reflect the behavior of the granitic rocks with respect to fracturing and weathering.

Quantitative estimation of petroleum prospect outcome probabilities: an overview of procedures

Abstract Estimation of prospect outcome probabilities in numerical form should be a central objective for petroleum geologists. Most numerical estimates of outcome probabilities represent subjective guesses. While the effectiveness of subjective procedures is difficult to gauge, several postmortem analyses suggest that subjective procedures are ineffective. Objective procedures for estimating outcome probabilities should yield much improved estimates. Objective procedures require that geological, geophysical and production data be organized so that geological and geophysical characteristics of prospects interpreted before they were drilled can be compared systematically with outcomes of prospects after they have been drilled. An example application that involves well data and an example application that involves seismic data demonstrate that objective procedures are simple in principle, but require organization of information in a form suitable for computation of frequencies, on which objective estimates of probabilities can be based.

Sedimentary Process Simulation: A New Approach for Describing Petrophysical Properties in Three Dimensions for Subsurface Flow Simulations

Subsurface fluid flow is critically dependent on the 3D distribution of petrophysical properties in rocks. In sequences of sedimentary rocks these properties are strongly influenced by lithology and facies distribution that stem from the geologic processes that generated them. Three types of simulators are contrasted that represent variations of petrophysical properties: stochastic simulators, stratigraphic-form simulators, and sedimentary process simulators. The first two generally require closely spaced well information or seismic data and can be “conditioned” to accord with the data, but neither can represent the influence of depositional processes directly. By contrast, process simulators do not require closely spaced data, but they generally cannot be forced to accord closely with the data. The sedimentary process simulator described here, known as SEDSIM, provides 3D geometric forms and spatial distributions of grain sizes of alluvial, fluvial, and deltaic deposits that are controlled by information supplied as initial and boundary conditions, including the initial topographic surface, and fluvial and sediment discharge volumes through time. The resulting sediment distributions can be compared directly with maps and sections based on well data and seismic surveys, but they can also be transformed into estimates of porosity and permeability, thereby placing them in form for direct use with subsurface flow simulators. Several recent applications of SEDSIM in generating descriptions of groundwater aquifers and hydrocarbon reservoirs and their use in subsurface flow simulations are presented. Comparisons between complex actual and simulated 3D sequences suggest that statistical descriptions if simulated sequences could be used as input to stochastic simulators. This would combine the advantages of stochastic simulators that can condition simulations to field data, with the advantages of process simulators that treat geometric forms and flow properties of sequences interdependently and represent the development of sedimentary facies through space and time.

Mississippian Bioherms in Northeast Oklahoma

In northeast Oklahoma, the Mississippian Boone formation contains numerous striking bioherms composed of crinoidal debris. Most of the exposed bioherms occur in the St. Joe, or lowest member of the Boone formation. The Reeds Spring, or middle member, contains no bioherms, and the Keokuk, or upper member, contains a few known bioherms. The bioherms occur in widespread, thin-bedded crinoidal limestones and are related in origin to the thin-bedded limestones. Each bioherm consists of a lens-shaped core of massive limestone which is surrounded by and interfingers with thin-bedded limestones. The massive cores are 10-40 feet thick and 50-1,000 feet wide. The thin-bedded limestones dip steeply away from the massive cores. The bioherms are interpreted as having been formed by the accentuated growth and accumulation of crinoids. The bioherms formed mounds which were elevated as much as 50 feet above the floor of a shallow sea. The massive bioherm cores were probably created by the continuous deposition of crinoidal debris; whereas the bedded limestones represent deposition which was interrupted from time to time. The thin-bedded limestones on the flanks of bioherms were deposited on slopes as steep as 45°. A debris-binding agent, such as inorganically precipitated calcium carbonate or calcareous algae, is postulated to have bound the deposits of crinoidal debris together, permitting them to persist on such steep slopes. The bioherms are considered to have been organic reefs because they formed wave- esistant organic structures which formed elevated prominences in a shallow sea.

Origin, distribution and age of high-level chert gravels (Plio-Pleistocene) in eastern Kansas

Abstract It is probable that the rounded flint has partly originated from these [Flint] hills. G.C. Broadhead (1880). High-level (upland) chert [flint] gravels occur in almost all of the counties in the Osage Cuesta country of eastern Kansas. The locations of the high-level chert gravel deposits now are well known as a result of recent remapping of the state on a scale of 1:24:000, but the origin and age of the chert gravels remain as debatable today as they were 125 years ago. The gravels are composed of brownish, smooth, angular to subangular chert pebbles and cobbles ranging up to 6 inches (15 cm) in size. The chert clasts have been derived from Lower Permian limestones that contain abundant chert [flint] and crop out in the topographically high Flint Hills in east-central Kansas. The chert weathers from the limestones to create lag gravels, which in turn are reworked and transported eastward and southeastward by streams. In the glaciated region in northeastern Kansas, remnants of high-level chert gravel deposits occur in association with glacial tills, outwash, and erratics. In places the chert gravels occur beneath glacial deposits. South of the glaciated region, however, the remnants of the high-level gravels occur on bedrock surfaces whose elevations range from 20 to 200 feet (6 to 60 m) above the floodplains of major streams. The chert itself that forms the gravels is known to be Early Permian in age, based on fossils preserved in the pebbles and cobbles. Deposition of the high-level gravels, however, cannot be dated directly, but information bearing on their relative ages is provided by the degree of soil development over them and their elevations with respect to modern floodplains. Deposits at higher elevations may be Pliocene, whereas deposits at lower elevations may be early to middle Pleistocene. Chert gravels that occur on low-lying floodplains and in modern alluvium and are judged to be late Pleistocene to Recent in age.

Statistical Appraisal of Seismic Prospects in Louisiana-Texas Outer Continental Shelf

One of the most important functions of the petroleum geologist is appraisal of prospects in advance of drilling. Almost invariably this involves predicting the amount of oil that may be discovered using only indirect evidence such as seismic maps and profiles. A widely used appraisal method, the Monte Carlo technique, requires geologic information that cannot be known prior to drilling, necessitating the use of estimates that may be highly uncertain. The results of Monte Carlo analysis may be misleading if the assumed geologic parameters, such as reservoir thickness, structural closure, and percent of fill, are not accurate. Simple statistical procedures based on regressions between seismic characteristics that are discernible before drilling and the volumes of oil and ga subsequently discovered in prospects provide an alternative objective basis for prospect appraisal. From a regression, an estimate can be made of the probability distribution of oil or gas field sizes associated with prospects of specified characteristics. The resulting distribution can be used directly for prospect appraisal purposes, or can be combined with engineering and financial data to yield an expected monetary value which incorporates elements of risk and uncertainty. To demonstrate the method of statistical appraisal, structural prospects were evaluated in part of the Pleistocene trend of offshore Louisiana and Texas, an area of about 3 million acres (1.2 million ha.). Highly significant statistical relationships were found between structural properties measured on prospects shown on regional seismic reconnaissance maps, including area of structural closure, and the volumes of oil and gas subsequently discovered by drilling these prospects. A comparison with conventional Monte Carlo evaluations of tracts in the Gulf Coast outer continental shelf suggests that statistical procedures based on seismic information may be more than twice as effective as Monte Carlo methods employing subjectively estimated reservoir characteristics. It may be possible, owever, to improve the performance of Monte Carlo methods by replacing the estimates of reservoir characteristics with statistically predicted reservoir volumes.

Chapter 7 Carbonate Oil Reservoir Rocks

Summary Half or more of the worlds petroleum is produced from carbonate reservoir rocks. The oil-reservoir characteristics of carbonate rocks are largely functions of porosity and relative permeability, which, in turn, have been affected by initial composition of the rocks and their subsequent history. Porosity of carbonate rocks may be arbitrarily divided into (1) primary porosity (formed during deposition), (2) secondary porosity (formed by solution, fracturing or other changes after deposition), and (3) sucrose dolomite porosity (resulting from replacement of calcite by dolomite). Primary porosity may in turn be subdivided into (a) framework porosity resulting from pores that remained as a result of the “sheltering” effect of rigid or loosely-aggregated frameworks, (b) mud porosity, consisting mostly of minute pores that remained in partly compacted carbonate mud that was subsequently lithified, and (c) sand porosity consisting of voids between sorted sand and gravel-sized carbonate particles. Most primary pores have been modified by solution (and cementation). Consequently, there is no sharp dividing line between primary porosity and secondary porosity resulting from solution. Sucrose dolomite porosity is important in many oil reservoirs. In these rocks, porosity and permeability have been strongly influenced by composition of the original carbonate sediment and the degree to which the rock has been replaced by sucrose dolomite. For example, in certain Devonian rocks in west Texas, which originally consisted of varying proportions of lime mud and of crinoid-stem fragments, the greatest porosity occurs in rocks that have been most highly dolomitized. Here, the percentage of dolomite tends to be greatest in rocks which originally contained about 45% lime mud and 55 % crinoid-stem fragments. The performance of carbonate reservoirs depends to a substantial degree on shapes and dimensions of pores and their geometric arrangement with respect to each other. Under oil-reservoir conditions, pores in rocks are generally occupied by either water or oil. Ordinarily, the reservoir rock is water wet, that is, each rock grain is surrounded by a thin film of water, and oil is generally the non-wetting phase. Isolated oil globules ordinarily will not migrate through the rock because the interfacial tension between water and oil is so high that the globules will not pass through the throats of pore interconnections. Before the oil can move as a separate phase, the displacement pressure between the oil-water interface must exceed the entry pressure of the pore interconnections. The displacement pressure is influenced principally by buoyancy, whereas entry pressure depends on the interfacial tension between water and oil, and on pore geometry. The minimum height of an oil column necessary for buoyant rise through a water-wet carbonate rock thus partly depends on the diameters of throats of pores and diameters of the interiors of pores. The reservoir performance of carbonate rocks may be predicted by injecting mercury into cores from reservoirs. Mercury, a non-wetting fluid, is forced into the core sample under increasing pressure. A graph of the data, showing injection pressure, versus cumulative volume of mercury injected, is an effective guide to the conditions required for oil to move in the rock. Ancient depositional environments exert strong influence on carbonate deposits formed in them, and, in turn, have subsequent effect on oil-reservoir conditions in carbonate rocks. Many examples could be cited. In Mississippian carbonate reservoirs in southeastern Saskatchewan, oolitic and pseudo-oolitic limestones interpreted to have been formed through chemical precipitation in a barrier bank environment, serve Iocally as excellent, highly permeable oil reservoirs. In west Texas, the Pennsylvanian-Permian Horseshoe atoll is a horseshoe-shaped mass of limestone about 90miles across in an east-west direction and 70 miles from north to south. It is interpreted to be analogous to modern reef atolls of the East Indies. In the Paradox Basin of southeastern Utah, limestone lenses composed largely of leaflike calcareous Algae serve as oil reservoirs. In Alberta, much oil is produced from Devonian rocks in which favorable reservoir conditions are closely associated with stromatoporoids and calcareous Algae. The geographic outlines of certain oil fields in Alberta, such as Red-water field, are essentially parallel to the trends of ancient organism communities. Thus, there is strong incentive to interpret ancient carbonate environments and organism communities, and to understand their effects on oil-reservoir properties.