Porosity In Dolostones Research Articles

Dolomite porosity related to peritidal microbialites textures and facies succession in Mesoproterozoic Wumishan Formation, North China

There are three main types of microbial dolostones in Mesoproterozoic Wumishan Formation in Jixian, North China. They are stromatolite, thrombolite and granular microbialite. The microbialite lithofacies are intensely environmentally associated. Several kinds of stromatolite can be found in the peritidal environments. Mostly planar or wavy stromatolites show in intertidal and supratidal zones while domy and conic stromatolites occur in subtidal or lower intertidal zones. In more agitated subtidal zone or tidal channels, microbial grains deposited, such as thrombolite, oncolite, and sand- gravel sized microbial-coated grains. The intraclasts related laminite facies mostly deposited in shallow subtidal zone.Representative four outcrop sections of Wumishan Formation in Jixian are measured and documented. The petrophysical and petrological study from the outcrop and well core samples reveals that the reservoir rocks are closely related to lithofacies and their depositional environments. The more agitated the depositional environment is, the better the reservoir quality, even so that after the diagenesis (surface and burial). The dolostone porosity of planar stromatolite in supratidal and intertidal are much worse than the subtidal high-agitated microbialite rocks, whether samples from the outcrops or drilling cores.The effective reservoir interval is normally located in the lower part of a peritidal microbial facies succession, mainly consists of subtidal microbialite, such as thrombolite, thrombo-stromatolite, oncolite, columnar stromatolite and coated intraclastic grainstone and rudstone, commonly deposited in agitated environment. The thickness of reservoir rocks in microbialite para-sequences ranges commonly from 4 to 14 m, deeply related to depositional cycles or sequence stratigraphy in vertical lithofacies texture and their areal extent to the paleogeography.

Multiphase Diagenetic Processes and Their Impact on Reservoir Character of the Late Triassic (Rhaetian) Kingriali Formation, Upper Indus Basin, Pakistan

Multiple episodes of dolomitization of the shallow marine carbonates of the Late Kingriali Formation resulted in regional scale mappable dolostone geobodies in the Kohat and Potwar sub-basins. With the exception of few unaltered patches of the host limestone, more than 90% of the carbonates of the studied formation are diagenetically altered by replacive dolomites with associated dolomite cementation. Petrographical and geochemical data interpretation reveals that during the initial stage of dolomitization, the precursor limestone was significantly modified by the fabric-retentive replacive dolomite (RD-I) and produced bulk dolostones with non-planar-a to planar-s crystals. Neomorphic recrystallization (RD-II) was observed as an overgrowth of the already formed RD-I dolomite crystals during progressive dolomitization. The seawater at shallow depths is enriched with Fe-ions due to its interaction with Fe-rich beds within the studied formation. The modified seawater actively participated in the formation of ferroan replacive dolomites (RD-III). Stable isotopic composition of the unaltered Echinoderm plates, calcite cement (CC-I), and RD-I demonstrates signatures of δ18O and δ13C within the limit of late Triassic marine seawater or modified seawater. Depletion in the stable oxygen isotopic composition (from −0.99‰ to −3.75‰ V-PDB) demonstrates that RD-II and RD-III were formed in a sequence with progressively higher temperature fluids than normal seawater. Precipitation of dolomite cements as cavity filling rhombs (DC-I) and crystal overgrowth (DC-II) with highly depleted δ18O values (−5.44‰ to −7.45‰ V-PDB) illustrates dolomite cementation at higher temperatures and greater depths. The highly depleted values of δ18O (up to −9.16‰ V-PDB) and (up to 0.42‰ V-PDB) for δ13C of saddle dolomite (SD-I) indicate the precipitation of SD-I as a cavity filling dolomite at considerable depth. Calcite cementation and calcitization actively participated in the early, middle, and late diagenetic modifications as interpreted from their petrographic and stable isotopic studies. Porosity enhancement is clearly demonstrated by dissolution, stylolization, fracturing, and replacement dolomitization. Dolomite and calcite cementation had a negative impact on the reservoir character and occluded the dolostone porosity to a greater extent.

Creation of Secondary Porosity in Dolostones by Upwelling Basement Water in the Foreland of the Alpine Orogen

Creation of secondary dissolution porosity in carbonate rocks during deep burial has the potential to improve reservoir properties for hydrocarbons, gas storage, and geothermal applications. However, the occurrence and mechanisms of such porosity enhancement are controversial. Here, we present compelling evidence for generation of deep burial porosity from the Swiss Molasse Basin, where dissolution of eogenetic anhydrite nodules in dolostones of the Middle Triassic Muschelkalk increased the matrix porosity by up to 15 vol.%. We reconstruct the genesis and evolution of the anhydrite-dissolution porosity based on petrography, porosity determinations, analyses of stable and radiogenic isotopes (δ2H, δ18O, δ34S, and 87Sr/86Sr), fluid inclusion studies, and laser U–Pb geochronology of secondary calcite. The results show that modified meteoric waters derived from the Variscan crystalline basement ascended via basement–cover cross-formational faults into the overlying Muschelkalk, where they dissolved the anhydrite nodules throughout an area of at least 55 km2 at 700–2300 m depth and 40–160°C. Secondary calcite in anhydrite moulds yields Late Eocene to Middle Miocene U–Pb ages, which coincide with the timing of basement uplift in the foreland bulge of the Swiss Alpine Orogen. This uplift provided the hydraulic gradients to drive meteoric water deep into the adjacent Molasse basin. Similar enhancement of reservoir properties can be expected in dolostones in other foreland basins that are bordered by a foreland bulge in which fractured basement rocks are exhumed.

Acoustic televiewer amplitude data for porosity estimation with application to porewater conversion

The ability to detect and quantify rock matrix porosity is explored using acoustic televiewer images and dolostone rock core samples. In particular, an empirical relationship between the acoustic televiewer amplitude response and core-derived total porosity is developed. The performance of the acoustic televiewer amplitude relationship was compared to neutron and gamma-gamma logs, which are traditional nuclear wireline tools commonly used for estimating porosity in dolostones. Acoustic televiewer amplitude results are consistent with the gamma-gamma (density) and neutron (porosity) logs, provide increased vertical resolution, do not require downhole radioactive sources and also provide oriented fracture information. Sample specific porosity and bulk density values from the calibrated acoustic televiewer datasets are then used to improve porewater concentration estimates from rock concentrations.

Acoustic televiewer amplitude data for porosity estimation with application to porewater conversion

Abstract The ability to detect and quantify rock matrix porosity is explored using acoustic televiewer images and dolostone rock core samples. In particular, an empirical relationship between the acoustic televiewer amplitude response and core-derived total porosity is developed. The performance of the acoustic televiewer amplitude relationship was compared to neutron and gamma-gamma logs, which are traditional nuclear wireline tools commonly used for estimating porosity in dolostones. Acoustic televiewer amplitude results are consistent with the gamma-gamma (density) and neutron (porosity) logs, provide increased vertical resolution, do not require downhole radioactive sources and also provide oriented fracture information. Sample specific porosity and bulk density values from the calibrated acoustic televiewer datasets are then used to improve porewater concentration estimates from rock concentrations.

Depositional, sedimentary, and diagenetic controls on reservoir quality in carbonate successions: A case study from the carbonate gas reservoirs of the Lower Triassic Feixianguan Formation, eastern Sichuan Basin, China

The Kaijiang–Liangping trough is an important carbonate gas zone in eastern Sichuan Basin. The western platform of this trough has received less attention than the eastern one due to its lower reservoir potential. The current study mainly investigated depositional, sedimentological, and diagenetic features of the Lower Triassic Feixianguan Formation (T1f) from the Tieshan area located at the western platform, aiming to discuss their controls on reservoir quality. The gentle slope in the Tieshan area (approximately 5°) deposited platform marginal oolitic shoals featured by thin vertical accumulation and long-distance lateral migration, thereby forming small-scale and scattered-distribution reservoirs. Despite intensive diagenesis alterations after sedimentation, the T1f reservoir porosity was controlled by sedimentological factors (i.e., grain size and sorting) at varying degrees. High-quality reservoirs occurred in the dolograinstones with good sorting and coarse grain sizes. The T1f carbonates underwent the near-surface and burial diagenesis in the Tieshan area. Dolomitization and associated syn- and post-dolomitizational dissolutions were important factors for secondary porosity development. Dolomite recrystallization increased intercrystal porosity in dolostones characterized by precursor mud-dominated fabric, thereby resulting in gradual shift from Lucia class 3 to class 1 petrophysical relationships with increasing crystalline sizes. By contrast, the intergrain porosity was not influenced by recrystallization in dolograinstones, which pervasively displayed classes 1 and 2 petrophysical relationships regardless of dolomite crystal sizes. The T1f gas reservoirs evolved from a paleo-oil accumulation as evidenced by pore-filling bitumen, which had negative effects on crystalline dolostones by filling intercrystal pores. However, its influence on dolograinstones was negligible as dissolution in relation to thermochemical sulfate reduction (TSR) reaction occurred during paleo-oil cracking into gas, thereby increasing intergrain porosity and effectively offsetting porosity decrease caused by bitumen filling. TSR dissolution occurred at certain intensity in the Tieshan area, which was evidenced by H2S occurrence in gas.

Paleo-watertable definition using cave ferromanganese stromatolites and associated cave-wall notches (Sierra de Arnero, Spain)

The steeply-dipping-dolostone-hosted caves of the Sierra de Arnero (N Spain) contain low-gradient relict canyons with up to ten mapped levels of ferromanganese stromatolites and associated wall notches over a vertical range of 85m, the highest occurring ~460m above base level. Despite a plausible speleogenetic contribution by pyrite oxidation, and the irregular cave-wall mesomorphologies suggestive of hypogenic speleogenesis, the Arnero relict caves are dominantly epigenic, as indicated by the conduit pattern and the abundant allogenic sediments. Allogenic input declined over time due to a piracy-related decrease in the drainage area of allogenic streams, explaining the large size of the relict Arnero caves relative to the limited present-day outcrop area of the karstified carbonates. Allogenic-sediment input also explains the observed change from watertable canyons to phreatic conduits in the paleo-downstream direction. Stromatolites and notches arguably formed in cave-stream passages at the watertable. The best-defined paleo-watertables show an overall slope of 1.7°, consistent with the present-day relief of the watertable, with higher-slope segments caused by barriers related to sulfide mineralization. The formation of watertable stromatolites favored wall notching by the combined effect of enhanced acidity by Mn–Fe oxidation and shielding of cave floors against erosion. Abrasive bedload further contributed to notch formation by promoting lateral mechanical erosion and protecting passage floors. The irregular wallrock erosional forms of Arnero caves are related partly to paragenesis and partly to the porous nature of the host dolostones, which favored irregular dissolution near passage walls, generating friable halos. Subsequent mechanical erosion contributed to generate spongework patterns. The dolostone porosity also contributes to explain the paradox that virtually all Arnero caves are developed in dolostone despite being less soluble than adjacent limestone. U-series dating of carbonate speleothems and paleomagnetic data from ferromanganese stromatolites and clastic sediments indicate that the paleo-watertables recorded ~320m above the present-day watertable formed during the Matuyama Chron but prior to ~1.5Ma, implying long-term base-level-lowering rates from ~125 to ~213m/Ma. To our knowledge, this is the first attempt of paleomagnetic dating of cave ferromanganese stromatolites. These deposits are excellent geomagnetic recorders and offer a direct way to delineate and date paleo-watertables, especially in caves developed in dolostone.

Dolomitization process and its implications for porosity development in dolostones: A case study from the Lower Triassic Feixianguan Formation, Jiannan area, Eastern Sichuan Basin, China

Abstract High-quality reservoirs in the third member of Lower Triassic Feixianguan Formation in the Jiannan area are exclusively restricted to dolostone, which makes it essential to understand origin of porosity in dolostone. However, the studies on the porosity origin of the dolostone in the Jiannan area are rare, and its origin is still controversial. In the paper, the formation mechanisms of porosity in replacement dolostones ever proposed worldwide are summarized, and they can be divided into two main categories based on the formation time of porosity relative to the dolomitization process: syndolomitization and postdolomitization origin. Studies on well cores that have been dolomitized to varying degrees indicate that the porosity in dolostones in the Jiannan area is a syndolomitization origin which is intimately associated with the dolomitization process, depending mainly on the degrees of dolomitization. Four stages of porosity evolution, each with distinctive petrographic and petrophysical properties, were distinguished. In initial stage of dolomitization (stage I), dolomite was formed mainly by volume for volume replacement with porosity constant or slight decrease due to external carbonate source added. In the next stages of progressive dolomitization (stage II), only minor intercrystalline pores were developed and the porosity slightly increased. In more advanced stage (stage III), abundant intercrystalline, as well as some vuggy pores, were formed by selectively dissolution of residual microcrystalline calcites and micritized grains, and the porosity remarkably increased with increasing dolomite content. In the most advanced stage (stage IV), dolomite cements were precipitated in pre-exiting pores in dolostone, resulting in varying decrease in porosity and permeability. Carbonates that must proceed at least in the stage III have the high porosity, while the dolostones in the stage II-2 that display similar rock frameworks only have a minor increase in porosity.

Fault-controlled and stratabound dolostones in the Late Aptian–earliest Albian Benassal Formation (Maestrat Basin, E Spain): Petrology and geochemistry constrains

Fault-controlled hydrothermal dolomitization of the Late Aptian to earliest Albian Benassal Fm shallow water carbonates resulted in the seismic-scale stratabound dolostone geobodies that characterize the Benicàssim case study (Maestrat Basin, E Spain). Petrological and geochemical data indicate that dolomite cement (DC1) filling intergranular porosity in grain-dominated facies constituted the initial stage of dolomitization. The bulk of the dolostone is formed by a replacive nonplanar-a to planar-s dolomite (RD1) crystal mosaic with very low porosity and characteristic retentive fabric. Neomorphic recrystallization of RD1 to form replacive dolomite RD2 occurred by successive dolomitizing fluid flow. The replacement sequence DC1-RD1-RD2 is characterized by a depletion in the oxygen isotopic composition (mean δ18O(V-PDB) values from −6.92, to −8.55, to −9.86‰), which is interpreted to result from progressively higher temperature fluids. Clear dolomite overgrowths (overdolomitization) precipitated during the last stage of replacement. Strontium isotopic composition suggests that the most likely origin of magnesium was Cretaceous seawater-derived brines that were heated and enriched in radiogenic strontium and iron while circulating through the Paleozoic basement and/or Permo-Triassic red beds. Burial curves and analytical data indicate that the replacement took place at burial depths between 500 and 750 m, and by hydrothermal fluids exceeding temperatures of 80 °C. Following the partial dolomitization of the host rock, porosity considerably increased in dolostones by burial corrosion related to the circulation of acidic fluids derived from the emplacement of the Mississippi Valley-Type deposits. Overpressured acidic fluids circulated along faults, fractures and open stylolites. Saddle dolomite and ore-stage calcite cement filled most of the newly created vuggy porosity. Subsequent to MVT mineralization, precipitation of calcite cements resulted from the migration of meteoric-derived fluids during uplift and subaerial exposure. This late calcite cement destroyed most of the dolostone porosity and constitutes the main cause for its present day poor reservoir quality.

The porosity origin of dolostone reservoirs in the Tarim, Sichuan and Ordos basins and its implication to reservoir prediction

Origin of dolostone remained a controversial subject, although numerous dolomitization models had been proposed to date. Because of the dolomitization’s potential to be hydrocarbon reservoirs, one debatable issue was the role of dolomitization in porosity construction or destruction. Based upon case studies of dolostone reservoirs in various geological settings such as evaporative tidal flat (Ordos Basin, NW China), evaporative platform (Sichuan Basin, SW China), and burial and hydrothermal diagenesis (Tarim Basin, NW China), here we systematically discuss the origin of porosity in dolostone reservoirs. Contrary to traditional concepts, which regarded dolomitization as a significant mechanism for porosity creation, we found two dominant factors controlling reservoir development in dolostones, i.e., porosity inherited from precursor carbonates and porosity resulted from post-dolomitization dissolution. Actually, dolomitization rarely had a notable effect on porosity creation but rather in many cases destroyed pre-existing porosity such as saddle dolostone precipitation in vugs and fractures. Porosity in dolostones associated with evaporative tidal flat or evaporative platform was generally created by subaerial dissolution of evaporites and/or undolomitized components. Porosity in burial dolostones was inherited mostly from precursor carbonates, which could be enlarged due to subsequent dissolution. Intercrystalline porosity in hydrothermal dolostones was either formed during dolo- mitization or inherited from precursor carbonates, whereas dissolution-enlarged intercrystalline pores and/or vugs were usually interpreted to be the result of hydrothermal alteration. These understandings on dolostone porosity shed light on reservoir prediction. Dolostone reservoirs associated with evaporative tidal flat were laterally distributed as banded or quasi-stratified shapes in evaporite-bearing dolostones, and vertically presented as multi-interval patterns on tops of shallowing-upward cycles. Dolostone reservoirs associated with evaporative platform commonly occurred along epiplatforms or beneath evaporite beds, and vertically presented as multi-interval patterns in dolostones and/or evaporite-bearing dolostones of reef/shoal facies. Constrained by primary sedimentary facies, burial dolostone reservoirs were distributed in dolomitized, porous sediments of reef/shoal facies, and occurred vertically as multi-interval patterns in crystalline dolostones on tops of shallowing-upward cycles. Hydrothermal dolomitization was obviously controlled by conduits (e.g., faults, unconformities), along which lenticular reservoirs could develop.

GEORGINA BASIN—AN EARLY PALAEOZOIC CARBONATE PETROLEUM SYSTEM IN QUEENSLAND

Queensland contains a number of carbonate-bearing basins which are under-explored for petroleum, but contain the elements of potentially economic petroleum systems. The oldest such basin is the Neoproterozoic to Ordovician Georgina Basin which straddles the Queensland-Northern Territory border and is traversed by the Ballera to Mount Isa gas pipeline.The basin developed across several major crustal blocks resulting in regional variations in deposition and deformation. Thick Neoproterozoic rocks of the Centralian Superbasin form the base of the sequence in apparently fault-bounded, extensional sub-basins. These rocks are generally tight and source rocks are unknown. The Cambrian to Ordovician rocks have the best petroleum potential with the most prospective part of the basin being the Toko Syncline. The Burke River Structural Belt is less prospective, but is worthy of further exploration. Basin fill consists of Cambrian and Early Ordovician rocks which are dominantly carbonates, with both limestones and dolostones present. In the Early to Middle Ordovician, the rocks became predominantly siliciclastic.The main phase of deformation affecting the Georgina Basin occurred in the Devonian as part of the Alice Springs Orogeny. The Toomba Fault, which forms the western boundary of the asymmetric Toko Syncline, is a thrust fault with up to 6.5 km of uplift. The angle of thrusting is between less than 40 degrees and up to 70 degrees. Rich, marine source rocks of Middle Cambrian age in the Toko Syncline are mature for oil except in the deepest part of the syncline where they are mature for dry gas. The deeper part of the Toko Syncline may be gas saturated.Potential hydrocarbon targets include large folds associated with fault rollovers, stratigraphic traps and faultbounded traps. Vugular, secondary porosity in dolostones offers the best chance for commercial reservoirs within the Ninmaroo and Kelly Creek formations and Thorntonia Limestone. There are also oolitic carbonates which may have good primary porosity, as well as interbedded sandstones in the carbonates with preserved porosity. Structurally controlled hydrothermal dolomite facies represent potential reservoirs. The dominantly siliciclastic Ordovician sequence is water flushed. Fracture porosity is another possibility (cf. the Palm Valley gas field in the Amadeus Basin). As the deeper part of the Toko Syncline appears to be gas saturated, there may be potential for basin-centred gas. Fine-grained carbonates and shales provide excellent seals. There has not been a valid structural test; although AOD Ethabuka–1 flowed 7,000 m3/d of dry gas, the well was abandoned short of the target depth.

Porosity–permeability relationships in Miocene carbonate platforms and slopes seaward of the Great Barrier Reef, Australia (ODP Leg 194, Marion Plateau)

AbstractThis paper reports a series of 700 porosity–permeability analyses and supporting petrographic and sedimentologic descriptions from Early to Late Miocene carbonate strata cored on the Marion Plateau, offshore from north‐eastern Australia, during Ocean Drilling Program (ODP) Leg 194. The samples analysed are not only mainly coarse bioclastic limestones and dolomitized equivalents from platform‐top facies, but also include 79 plugs from deeper‐water slope to hemipelagic drift facies. Outstanding characteristics of this data set are the wide ranges of porosity and permeability in both limestones and dolostones, the large degree of short‐range heterogeneity typical of these strata, and the better porosity–permeability correlation of dolostones than limestones. The platforms have experienced widely varying calcite cementation, dolomitization and dissolution but show little clear evidence of meteoric diagenesis, suggesting that subaerial exposure may have played little role in porosity–permeability evolution. Permeability‐for‐given‐porosity is controlled by grain size and calcite cement content in grainstones and by occurrence of larger shelter pores and vugs in mud‐rich samples. Dolomitization tends to reduce the variation of permeability‐for‐given‐porosity by recrystallizing mud matrix to form intercrystalline macroporosity that connects vugs and moulds to become integrated with the effective pore system. As a result, there are no differences in permeability–porosity trends for different dolostone textures, whether dominated by intercrystalline, vuggy, or preserved intergranular pore types. Two platform‐top sites separated by only 5 km display a major lateral variation in dolostone porosity–permeability characteristics within the youngest dolostone units. This difference is interpreted as reflecting a relatively ‘windward’ (current‐facing) setting of the site with the overall higher permeability‐for‐given‐porosity (Site 1199) that led to less muddy depositional facies, greater cementation, and lesser grain dissolution. Pore‐geometry parameters measured by petrographic image analysis confirm that the ‘windward’ dolostones have pores that are both larger and less intricate than dolostones comprising the more current‐protected location.

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.

Sedimentological Cross Section of Cambro-Ordovician Carbonate Shelf (Knox Group, Conasauga Formation) in Central Alabama: Facies, Diagenesis, Potential Reservoirs: ABSTRACT

Cambro-Ordovician thrust-imbricated carbonates in central Alabama are the focus of renewed exploration interest. Samples from east-west-trending core holes within the surface-most thrust plates reconstruct the carbonate shelf and shelf-edge facies before deformation. The Upper Cambrian shelf margin now is in the subsurface of Talledega County; coeval dolostones in the western part of the state represent the former shelf interior. Rock analogs to former environments include the following. (1) Barrier shoals (Conasauga Formation)--dark colored, partially dolomitized ooid and skeletal grainstones. (2) Submerged back-barrier and offshelf dolomitized sediments (lower Knox Group)--western belt: finely crystalline algal thrombolites, fenestral dolopelmicrites, rippled beds; east rn belt: finely laminated dolostones, slope-derived pebbles and graded beds. (3) Tidal flats (upper Knox Group)--light-colored, crystalline dolostones, dolomitized pellet grainstones, algal laminites, pseudomorphs after sulfates and early diagenetic chertification. (4) Former emergent shelf-(Knox unconformity)--pelmicrite, skeletal wackestones, erosional chert pebble conglomerate. Multiple possibilities for hydrocarbon reservoirs appear throughout the sequence. Vuggy and intercrystalline dolostone porosity is primarily in the lower Knox formations. Primary interparticle pores are retained in lower Knox algal buildups. Breccia porosity occurs in the strata below the Knox unconformity through solution of the underlying Knox Group. Fractures in the subsurface are believed to enhance permeability in all porosity types. End_of_Article - Last_Page 532------------

Dolostone Reservoirs in Horquilla Formation (Pennsylvanian-Permian), Big Hatchet Peak Section, Hidalgo County, New Mexico: ABSTRACT

Dolostone reservoirs with net thickness of 484 ft (148 m) in upper Horquilla, are exposed in the Big Hatchet Peak section of southwestern New Mexico and constitute the best petroleum objective demonstrated to date in the Pedregosa basin. This shallow-marine carbonate section includes uppermost Paradise Formation (Chesterian) and 3,230 ft (985 m) of Horquilla (Morrowan-Wolfcampian). The lower member of the Horquilla consists of limestone and chert; a disconformity at the top formed during the Desmoinesian. The upper member is at least 1,867 ft (569 m) thick; limestones contain phylloid-algal biostromes and ancient solution cavities; alternating laterally extensive dolostones are 54 to 148 ft (16.5 to 45 m) thick and exhibit much vuggy porosity. Petrographic evidence demonstrates that the limestones were stabilized within freshwater diagenetic environments, that they contained much primary or secondary porosity, and that practically all of it was occluded by marine or freshwater cements. Dolostones also contained much secondary intercrystalline and moldic porosity that was partly occluded by epitaxial cements and coarse recrystallization of initial neomorphic rhombs. Anhydrite porphyroblasts were emplaced by hypersaline waters and were dissolved later by fresh ground waters to form molds with distinctive stairstep outlines; this tertiary (third stage) porosity in dolostones was partly filled by gravitational cements, and some microstalactite tips were dolomitized paramorphically. Dark Horquilla limestones are rated as fair petroleum-source units. Land-derived kerogens indicate sources of gas; alteration indices of 3 to 3+ indicate moderate thermal history. Dolostones in the upper member are rated as fair to good reservoir units. Matrix porosity and permeability are poor but should improve farther south along the shelf edge and on the slope into Alamo Hueco basin. End_of_Article - Last_Page 571------------