In this early limestone-classification scheme, Archie incorporates two elements: the texture of the matrix and the size of the visible pores. The texture provides information on the minute pore structure between the crystals, granules, or fossils and is derived from both hand sample and microscopic investigation at low power as is the pore size. These observations are utilized to derive three reservoir fabric classes — compact, chalky, and sucrose. Utilizing data from three different fields, each of which is representative of one of the three reservoir fabric classes, porosity-permeability cross plots and capillary pressure curves were derived. In addition, resistivity data were measured from cores from the three different fields. Although petrophysical relationships or trends in limestones are generally similar to those of sandstones, greater variations from the average are more typical of limestones because of their greater pore-structure heterogeneity. To aid in overcoming this obstacle to evaluating limestone reservoirs, Archie utilizes his reservoir fabric classification scheme in conjunction with the qualitative use of resistivity curves to show that net pay intervals in limestones may be identified by increases or decreases (or no variation at all) in resistivity, depending mainly on the variation of porosity (or permeability) with water saturation, i.e., pore-size distribution. This paper is the first attempt to understand and deal with the complexities of pore size and shape variations in limestones and their effects on reservoir evaluation; and it formed a foundation upon which later work, such as Lucia's (1995) rock-fabric/petrophysical classification was built.
This widely cited, classic paper, groups pores found in carbonate rocks into two major groups: "fabric selective" and "not fabric selective." Pore types in the fabric-selective group are characterized by their relationship to grains, crystals, and biogenic fabrics, yielding such pore types as interparticle for pores between grains; intercrystal for pores between crystals (e.g. dolomite crystals); and growth-framework for pores related to larger scale biogenic growth forms. Examples of not fabric selective pores include fractures and vugs. The basic pore types can be modified by using terms related to process (e.g., solution, cementation, internal sediment), direction or stage (enlarged, reduced, filled), time of formation (primary, secondary), pore size, and abundance. This classification scheme has proven extremely useful because it relates to depositional and diagenetic processes and is flexible and easy to apply. The authors also thoroughly describe the relationship between pore types, the original depositional fabric of the sediments, and the diagenesis that potentially occurs in three zones: eogenetic (early burial), mesogenetic (deeper burial), and late-stage diagenesis associated with erosion of long-buried carbonates (telogenetic). The Choquette and Pray pore-classification scheme has stood the tests of time and is the most widely used for carbonate reservoir rocks.
Hydrothermal dolomite (HDT) reservoirs are formed in the subsurface through alteration of a host rock (commonly limestone) by typically highly saline fluids with temperature and pressure higher than that of the host rock. Hence, these are diagenetically formed reservoirs and part of a spectrum of hydrothermal dolomite reservoirs that host sedimentary-exhalative lead-zinc ore bodies and Mississippi Valley-type sulfide deposits. Davies and Smith document a spectrum of HTD type reservoirs of many ages from around the world, noting their strong association with extensional and strike-slip faults. In these reservoirs, dolomitizing fluids have commonly been focused at transtensional and dilational structural sites and in the hanging wall, with transtensional sags above negative flower structures along wrench faults being favored drilling targets for potential HTD reservoirs. Characteristics of HTD reservoir facies are nicely documented with burial history plots, structure maps, geochemical data, core photographs, seismic data, and conceptual diagrams. This paper is a thorough survey of HTD reservoirs — their ages, locations, structural settings, and common characteristics. It offers "one-stop shopping" for anyone wanting an introduction to this enigmatic type of reservoir.
The late 1960s–1970s were a time of increasing recognition that carbonate-platform shedding and shelf-margin failure could result in significant volumes of coarse-grained, carbonate material being transported into a basin. In this paper Enos reinterprets the Tamabra limestone of the Poza Rica trend, a major reservoir with some 6.1 STBBOIIP and 6.9 TCFGIIP, as a succession of gravity-flow deposits, consisting of a heterogeneous mix of wackestones, packstones, grainstones, and breccias, situated basinward of the Golden Lane carbonate platform. This interpretation differs significantly from previous views that the Tamabra Limestone formed a lowstand shelf margin seaward of the Golden Lane platform margin or was the true shelf edge and the Golden Lane a faulted backreef-lagoon sequence. Keys to Enos' interpretation include, among other lines of evidence, the use of geopetals to document that rudists were not in growth position, graded beds, interlayering of coarse deposits and fine-grained limestone with pelagic microfossils, and seismic data to show that the Golden Lane platform was not fault bounded. This paper is significant because it opened up a new carbonate-reservoir play type by drawing on multiple lines of evidence to identify a major hydrocarbon accumulation as being housed in carbonates transported downslope into deep-water, rather than in a shallow-water carbonate platform.
The authors document the stratigraphic occurrence of fields producing from post-Ordovician carbonate buildups, noting that 90% of the approximately 40 BBOE of reserves found in carbonate buildups occur in just 15% of that time — a time also of extensive source-rock deposition. The authors also show that buildup growth has been favored within times of second-order accommodation increase, placing reservoirs, source rocks, and seals in favorable proximity to yield stratigraphic traps. Finally, the paper builds on the classification of carbonate reservoir types of Wilson (1980; also a highly regarded paper) and illustrates in a sequence-stratigraphic framework the trap types associated with carbonate reservoirs. The concepts presented by Greenlee and Lehmann are nicely illustrated and reinforce the paper's importance for risk assessment of carbonate trap types in general and productive carbonate buildups in particular.
This paper documents an approach to characterizing carbonate ramp reservoirs through the integration of sequence stratigraphic analysis, petrophysical quantification through definition of rock fabric (e.g. Lucia, 1995) flow units, and fluid flow simulation. Because the Permian San Andres Formation of the Permian basin is typical of carbonate-ramp reservoirs in general, in that it is highly stratified, has complex facies and permeability distribution, and generally low recovery efficiencies of 30% of original oil in place, the approach described by Kerans et al. has near-universal applicability to fields producing from this reservoir type. Some of the major findings of the study are that stratigraphy at the cycle (parasequence) scale is the essential correlation level for a petrophysically quantifiable framework; rock-fabric facies is the basic scaling level for petrophysical data within cycles; and fluid flow and reservoir performance are dependent on stacking of rock fabric facies within the cycle and sequence framework. The value of outcrop studies to understanding subsurface reservoirs is shown in this study and the fact that carbonate ramp reservoirs are hosts to some of the largest fields in the world, such as many in the Middle East, makes this paper especially important and useful.
Loucks weaves together studies of modern and ancient cave systems to derive a general model of the evolution of karst cave systems — from their near-surface inception to their ultimate collapse during subsequent burial — and the implications for hydrocarbon production from paleocave systems. The author describes the impact of both near-surface effects, including dissolution, cave fill, chemical precipitation, and local fracturing, brecciation and collapse, and burial effects such as cave-passage collapse and further brecciation on the spatial distribution of various pore types and their connectivity. Of particular note are the effects of paleocave collapse resulting in coalescence of paleocave systems; and differential compaction of rocks over and adjacent to collapsed passages leading to fractures and various types of breccias. These compactional effects have significant impact on pore types and reservoir connectivity and may extend well beyond the original cave passages. Cave and fabric types (breccias, etc.) are nicely illustrated through diagrams and photographs. A very useful compendium of paleocave systems and aspects of their dimensions and porosity are included in the paper as are a cumulative probability curve of passage width and an equation relating the maximum potential size of a brecciated zone resulting from passage collapse to the original width of the passage — all valuable information for anyone working with paleocave reservoirs.
Lucia's paper uses a modified Dunham (1962) carbonate fabric-classification scheme and modified Choquette and Pray (1970) pore classification terminology to link petrophysical properties with carbonate textures. Pore space is characterized as interparticle, intercrystal, and vuggy. In nonvuggy carbonate rocks, permeability and capillary properties are described in terms of particle size, sorting, and interparticle porosity. An important modification to the Dunham classification is the recognition that packstones may have intergrain pore space or cement (grain-dominated packstones) or intergrain spaces filled with mud (mud-dominated packstones). Vuggy pore space is divided into separate vugs and touching vugs on the basis of vug connectivity, with separate vugs being fabric selective and connected only through the interparticle pore network. Separate-vug porosity contributes little to permeability, whereas touching vugs are not fabric selective and form an interconnected pore system, but one not related to a depositional fabric and, therefore, one not applicable to the rock fabric/petrophysical approach. Permeability and saturation characteristics of interparticle porosity are grouped into three rock-fabric/petrophysical classes based on mud content and crystal size in the case of dolomites; and generic porosity-permeability transforms and water saturation, porosity, reservoir-height equations are derived for each of the classes. Utilizing the rock fabric/petrophysical classification approach facilitates a linkage between depositional facies and fabrics and petrophysical properties, thereby integrating geological observations with engineering data to provide a more robust input to reservoir simulation models.
This paper brings a large data set of carbonate porosity values derived from wireline-logged wells drilled in south Florida to bear on the questions of how does the volume of carbonate pore space evolve with burial depth and is there a difference between limestones and dolomites in that respect, and in so doing provides useful guidance on porosity decline with depth for those exploring for carbonate reservoirs. The rocks in the study range from Neocomian (lower Cretaceous) to Recent and depths range from the surface to approximately 5.5 km (18,000 ft). A plot of the entire data set shows an exponential porosity decline with depth; with porosity of surface rocks as high as 55% and porosity of rocks at burial depths below 5.5 km (18,000 ft) less than 5%. Porosity-curve trends of individual stratigraphic units show that younger units have a less steep porosity-decline curve (i.e., greater loss of porosity with depth) than older units. In addition limestones start out with greater porosity at the surface than dolomites, but their less-steep porosity-decline curve shows that by a burial depth of 1.7 km (5,600 ft) limestones have lost porosity at a faster rate than dolomites and, from there on down to greater depths, dolomites have greater porosity at a given depth
Ward et al. relate the excellent exposures of Upper Permian rocks in the Guadalupe Mountains of west Texas to their hydrocarbon-productive counter parts in the subsurface of the Permian basin of New Mexico and Texas. Maps show the fields producing from individual formation(s), tables list the fields and describe the trap types, and cross sections and block diagrams illustrate the relationships of depositional facies belts to reservoirs, traps, and seals. This single paper provides an excellent overview of factors controlling occurrences of the various carbonate hydrocarbon traps in a well-explored, major hydrocarbon province.