F. Jerry Lucia

Rock-Fabric/Petrophysical Classification of Carbonate Pore Space for Reservoir Characterization

This paper defines the important geologic parameters that can be described and mapped to allow accurate petrophysical quantification of carbonate geologic models. All pore space is divided into interparticle (intergrain and intercrystal) and vuggy pores. In nonvuggy carbonate rocks, permeability and capillary properties can be described in terms of particle size, sorting, and interparticle porosity (total porosity minus vuggy porosity). Particle size and sorting in limestones can be described using a modified Dunham approach, classifying packstone as grain dominated or mud dominated, depending on the presence or absence of intergrain pore space. To describe particle size and sorting in dolostones, dolomite crystal size must be added to the modified Dunham terminology. Lar er dolomite crystal size improves petrophysical properties in mud-dominated fabrics, whereas variations in dolomite crystal size have little effect on the petrophysical properties of grain-dominated fabrics. A description of vuggy pore space that relates to petrophysical properties must be added to the description of interparticle pore space to complete the petrophysical characterization. Vuggy pore space is divided into separate vugs and touching vugs on the basis of vug interconnection. Separate vugs are fabric selective and are connected only through the interparticle pore network. Separate-vug porosity contributes little to permeability and should be subtracted from total porosity to obtain interparticle porosity for permeability estimation. Separate-vug pore space is generally considered to be hydrocarbon filled in reservoirs; however, intragranular microporosity is composed of small pore sizes and may contain capillary-held connate water within the reservoir. Touching vugs are nonfa ric selective and form an interconnected pore system independent of the interparticle system.

Dolomitization of Recent and Plio-Pleistocene Sediments by Marine Evaporite Waters on Bonaire, Netherlands Antilles: ABSTRACT

End_Page 535------------------------------The carbonate rocks and sediments on the island of Bonaire in the Netherlands Antilles contain interesting examples of early dolomitization. A large flat area of Recent supratidal sedimentation exists on the south end of Bonaire, and in this area evaporation of sea water is depositing calcium carbonate and gypsum, which produces dense brines having large Mg/Ca ratios. Dolomite is found in most of the Recent supratidal sediments, and carbon-14 dates on the dolomite establish that the time since dolomitization has been less than 2,200 years. Textural evidence indicates that some of the dolomite was formed by replacing lime sediments. The dense brines produced by evaporation tend to flow downwards into the permeable sediments, and an analysis of the chemistry and hydrology of a hypersaline lake chosen for detailed study shows that downward drainage of brine having a Mg/Ca ratio of about 30 must be happening today. Examination of marine Plio-Pleistocene rocks on the north end of Bonaire shows large areas of dolomitization whose boundaries cut across the bedding. The field evidence is consistent with the hypothesis that this dolomite has been produced by the flow of dense brines from a supratidal area. The time required to produce the estimated volume of dolomite found in the Plio-Pleistocene rocks would be of the order of 105 years. End_of_Article - Last_Page 536------------

Integrated Characterization of Carbonate Ramp Reservoirs Using Permian San Andres Formation Outcrop Analogs

The San Andres Formation (Permian, Guadalupian) of the Permian basin is representative of carbonate ramp reservoirs in that it has highly stratified character, complex facies and permeability structure, and generally low recovery efficiencies of 30% of original oil in place. The approach used here to describe carbonate ramp reservoirs such as the San Andres Formation produces detailed reservoir models based on integration of sequence stratigraphic analysis, petrophysical quantification through definition of rock fabric flow units, and fluid flow simulation. Synthesis of these subdisciplines clarifies which aspects of the geologic-petrophysical model are most significant in predicting reservoir performance and ultimately in understanding the location of remaining oil satur tion. The San Andres Formation crops out along the Algerita escarpment, a long, oblique-dip, continuous shelf-to-basin exposure in the central Guadalupe Mountains. These outcrops provide a unique opportunity to study lateral relationships in geologic and petrophysical structure analogous to those occurring between wells in subsurface reservoirs. On the basis of sequence stratigraphic analysis, three scales of cyclicity are recognized: depositional sequences, high-frequency sequences, and cycles. Examination of the cycles in two detailed window areas provides a practical scale for petrophysical quantification and fluid flow simulation. An understanding of cycle position within the high-frequency sequence framework also provides predictive information. Petrophysical analysis revealed six rock fabric groups dominated by intergranular, separate vug, or dense intercrystalline pore types. Comparison of these rock fabric groups with facies descriptions produced a rock fabric flow unit model that honors the geologic structure of the cycle and sequence framework. Permeability data were averaged within rock fabric flow units using a geometric mean approach based on fine-scale fluid flow modeling of deterministic and stochastically generated permeability fields. Two-dimensional black oil fluid flow models illustrate that (1) major differences in sweep efficiency and fluid flow performance are predicted when linear interwell interpolations are compared with actual interwell-scale geologic structure as determined by outcrop geologic and petrophysical mapping, (2) an understanding of static geologic/petrophysical conditions provides only a partial understanding of reservoir performance defined by the interaction of these static properties and dynamic properties of fluid flow interaction within the flow unit architecture, and (3) because of the orderly distribution of high- and low-permeability facies within cycle stacks of high-frequency sequences, this larger scale of geologic description can give a reasonable first-order approximation of fluid flow patterns and early breakthrough. End_Page 181------------------------------

Diagenesis of a Crinoidal Sediment

ABSTRACT The Devonian crinoidal rocks in the Andrews South Devonan field, Andrews County, Texas, show a clear relationship between the original sediment and its diagenetic history. The limestone facies is composed of a supporting framework of crinoid fragments, interparticle lime mud, and rim cement. Laterally, it changes to dolomite, which was originally composed of a supporting framework of lime mud containing scattered crinoid fragments. The porosity in the limestone facies was formed by selective leaching of interpartical lime mud after the formation of rim cement. The porosity in the dolomite facies was formed during dolomitization and is generally related to the ratio of the lime mud to crinoid fragments; the dolomite samples with the highest porosities contain the highest amount of crin id fragments.

Oxygen isotope composition of Holocene dolomite formed in a humid hypersaline setting

Holocene dolomite forming in supratidal carbonate sediments of Bonaire, Netherlands Antilles, is precipitated from hypersaline fluids in which the Mg:Ca ratio is elevated by evaporation and gypsum precipitation. Kinetic effects and water-vapor exchange between the humid atmosphere and hypersaline brines limit the δ18O values of the brines to between +0.7‰ and +2.1‰ standard mean ocean water. The mean δ18O value of dolomite is +1.0‰ Peedee belemnite ( n = 8), which is lower than values of dolomite precipitated from hypersaline brines in less humid environments. Bonaire dolomite is also isotopically lighter than dolomites interpreted to have formed in normal marine waters and mixed marine and/or meteoric waters. Calculated fractionation between dolomite and calcite formed from Bonaire brines is between 1.5‰ and 3.5‰. The high end of this range is consistent with previously proposed mineralogic fractionations of 3‰ ±1‰ and 3.2‰ between dolomite and calcite.

Outcrop-constrained hydrogeological simulations of brine reflux and early dolomitization of the Permian San Andres Formation

Our hydrogeologic model tests the effectiveness of brine reflux as the mechanism for early dolomitization of the Permian San Andres Formation. Brine circulation is constrained by sequence-stratigraphic parameters and a heterogeneous distribution of petrophysical properties based on outcrop data. The model simulates accumulation of the San Andres platform and calculates fluid flow and solute transport in response to relative sea level fluctuations. It tracks porosity loss caused by compaction and the concomitant permeability feedback. The amount of dolomite potentially formed is calculated by means of a magnesium mass balance between brine and rock. Results show that (1) brine reflux is an effective mechanism to deliver magnesium to dolomitize large carbonate successions; (2) relative sea level–controlled transient boundary conditions result in intricate flow and salinity patterns that can generate irregular dolomite bodies with complex spatial distributions; (3) pervasive dolomitization can result from several short-lived reflux events by the amalgamation of brine plumes sourced in different locations and times; and (4) the model successfully recreates the dolostone and limestone patterns observed in San Andres outcrops.

Origin and petrophysics of dolostone pore space

Abstract The common claim that dolomitization creates 12% porosity is based on the mole-for-mole replacement equation. However, in the past 50 years, data have been collected demonstrating that dolomitization does not create porosity. Instead, dolostones inherit the porosity and fabric of the precursor limestone, and porosity is reduced by overdolomitization. The porosity of the precursor limestone depends on the diagenetic history up to the time of dolomitization. Data show that: (1) carbonates are born with high porosity and lose porosity gradually over a long period; and (2) mud-dominated fabrics compact more readily than grain-dominated fabrics. The problem of estimating the time of dolomitization is minimized by confining observations to young limestones and associated dolostones. Limited data from Holocene dolomitic sediments suggest no change in porosity with dolomitization. Study of Plio-Pleistocene carbonates in Bonaire, Netherlands Antilles, demonstrates that precursor limestones are more porous than dolostones. Limestones average 25% porosity, whereas dolostones average 11% porosity. Data from the Neogene of the Great Bahama Bank show that dolostones and adjacent limestones both have 40% porosity. Porosity studies of the Jurassic Arab D Formation show that dolostones and associated grain-dominated limestones have similar porosity ranges and that the decrease in porosity with increasing dolomitization results from compaction of the mud-dominated fabrics. These data suggest that porosity in dolostone is not created by a mole-for-mole replacement mechanism. Instead, dolostone porosity is: (1) inherited from the precursor limestone; and (2) occluded by the process of overdolomitization. Palaeozoic dolostones, however, are commonly more porous than juxtaposed limestones. The explanation for this observation is that limestones lose porosity through compaction and cementation, whereas dolostones resist compaction and retain much of their porosity. Permeability studies have demonstrated that dolomitization of grain-dominated limestones usually does not change porosity-permeability relationships. Instead, precursor limestone fabric controls pore-size distribution. The dolomite crystal size of a muddominated dolostone may, however, be larger than the carbonate mud size, improving the porosity-permeability relationship substantially. Hence, there is a predictable relationship between interparticle (grains or crystals) porosity, permeability, precursor grain size and dolomite crystal size.

Outcrop/Subsurface Comparisons of Heterogeneity in the San Andres Formation

An integrated outcrop and subsurface study of permeability variations in the San Andres formation demonstrates the extreme heterogeneity present in this economically important carbonate horizon. Permeability measurements were made with a field permeameter and were compared to subsurface core data. Geostatistical techniques were used to predict variability and scales of spatial correlation. Measured permeability showed substantial variability within units arranged in three correlation scales. Outcrop permeability data exhibited no marked permeability anisotropy in predicted spatial correlation length. Several scales of spatial variability have been observed in an outcrop section, with subsurface results in agreement.

Styles of heterogeneity in dolomitized platform carbonate reservoirs: Examples from the central basin platform of the permian basin, southwestern USA

Abstract The Permian Basin of West Texas and southeastern New Mexico, southwestern U.S.A., is the premiere oil basin of the United States. At discovery, reservoiors in this prolific province contained more than 100 billion barrels of oil, almost a quarter of all the discovered in the U.S. Almost half of this resource (43%) was contained in a single reservoir type, dolomitized platform carbonate. Dolomitized platform carbonates were deposited on shallow shelves fringing the basin and on a horst block, the Central Basin Platform, that divides the basin and separates sites of deep-water siliciclastic sedimentation in the adjacent subbasins. The Central Basin Platform hosts many large combined structural/stratigraphic trap reservoirs in dolomitized platform carbonates. These range in size up to four billion barrels of original oil in place. Integrated geoscience and engineering characterization of four of these fields; Dune, Emma, Penwell, and Taylor-Link, allows comparison of the styles and scales of heterogeneities that influence recovery in this reservoir type. Facies composition and architecture exert fundamental controls on paths of fluid movement during production. Principal facies are extensive, deep subtidal fusulinid wackestones that shoal upward into shallow-subtidal and intertidal packstones and grainstones in which the dominant productive facies are grainstone bars and shoals, and shorezone systems that are dissected by dip-oriented tidal systems. Rapid lateral facies changes together with highly cyclic shoaling sequences result in pronounced permeability variations both laterally and vertically in the section. Superimposed on the depositional framework is a multi-event diagenetic overprint. These carbonate reservoirs are thoroughly dolomitized and partly cemented by sulfates. A post-burial leaching event increased permeability in some parts of these rocks. Karst processes have a large affect on reservoir quality in the southern part of the Central Basin Platform. Even though these carbonate reservoirs have undergone substantial diagenetic modification depositional facies still exert the primary control on remaining, and in particular, mobile, oil saturation.

The Great Lower Ordovician Cavern System

Karsting and collapse brecciation in the Lower Ordovician carbonates have been recognized for many years. However, the time of cavern formation and the geochemical hydrology responsible is debatable. In this chapter, I intend to review pertinent literature and evaluate the evidence of the presence of paleocaverns, the time of their formation, the history of cavern collapse to form collapse breccias, and the relationship of collapse breccias to structure. I will not review chemical hydrology issues because a discussion on the geochemical environment is pertinent only after the time of cavern development has been adequately resolved. The most robust data sets come from outcrop studies. Outcrops with extensive exposures reviewed here are the El Paso Group in the Franklin Mountains, west Texas; the Pogonip Group in Nopah Range, southeastern California; and the St. George Group in Newfoundland. The Lower Ordovician outcrops in central Texas, the Mississippi Valley, Virginia, and the Arbuckle Mountains are also useful. Robust subsurface data sets include the Ellenburger Group of Texas; the Knox Group of Tennessee, Kentucky, and Ohio; and the Arbuckle Group of central Kansas. Core descriptions from the subsurface Arbuckle Group in Oklahoma and Arkansas are also helpful. The most convincing evidence of cavern formation is roof collapse, that is, evidence that breccia blocks are below their stratigraphic positions. The time of cavern formation is more difficult to ascertain and most commonly is based on the source of the infilling sediment by comparing lithologies and, in places, using biostratigraphy. The history of the collapse can be determined only in the most extensive outcrops and mining operations, although modern three-dimensional (3-D) seismic volumes are useful. The relationship of collapse breccias to structure is basically a timing issue and can be resolved only by detailed geologic studies. I conclude from reviewing published data that convincing evidence shows that an extensive cavern system existed in the Lower Ordovician carbonates at the time of the Sauk-Tippecanoe unconformity. In some areas, the unconformity surface is highly irregular and appears to represent karst terrain. Caverns located far below the unconformity were most likely formed in response to internal disconformities. Lower Ordovician fractures and faults can have a controlling influence on the location and geometry of the caverns. Collapse of these caverns produced the collapse breccia and fracturing of the cavern roof. In some instances, cavern collapse has produced structural sags similar to those produced by the expansion related to strike-slip faulting. In extreme cases, collapse of large caverns produced breccia pipes that extended more than 330 m (1000 ft) into overlying Ordovician, Silurian, and possibly Devonian units.