Generation Of Secondary Porosity Research Articles

An integrated analytical approach for determining the origin of solid bitumens in the McArthur Basin, northern Australia

An integrated analytical approach using petrological, geochemical and isotopic techniques has been used to determine the origin of solid bitumens in the Middle Proterozoic Roper Group, McArthur Basin, northern Australia. Sandstone and dolerite samples, which in hand specimen contain dark material believed to be solid bitumens, were examined from six wells. Under ultraviolet illumination, non-fluorescent solid bitumens can be distinguished from two generations of crude oil, one of lower maturity (orange fluorescence), the other of higher maturity (white fluorescence). Other petrographic observations indicate that the precursor hydrocarbons to the solid bitumens and the subsequent charges of crude oil postdate all diagenetic clay and quartz cementation. Geochemical analyses indicate that many of the sandstones contain both an aliphatic-rich oil and solid bitumen, mixed in varying proportions, thus corroborating the petrological results. On the basis of n-alkane distributions, the oil appears to have two 3ubtly-different source inputs, one of which is typified by a greater proportion of high molecular weight n-alkanes which have a slight even carbon-number predominance and an isotopically lighter aliphatic hydrocarbon fraction. One sandstone sample contains solid bitumen in a fracture which is characterized by a high abundance of asphaltenes, polar compounds and sulphur and may have formed by deasphalting of a precursor oil. The solid bitumens recovered from fractures in dolerite sills were formed by thermal processes which in one sample resulted in an unusual series of ketone isomers. However most solid bitumens, particularly where present in the coarser sandstones, are related to the generation of secondary porosity. The large proportion of unresolved complex mixture, the lack of n-alkanes and the isotopically heavier aliphatic hydrocarbon fractions of samples containing predominantly solid bitumens indicate that bitumen formation occurred mainly by biodegradation of a precursor oil.

Solubility of petroleum in oil-field waters as a function of the oxidation state of the system

Research Article| September 01, 1994 Solubility of petroleum in oil-field waters as a function of the oxidation state of the system Vitalii A. Pokrovskii; Vitalii A. Pokrovskii 1Department of Geology and Geophysics, University of California, Berkeley, California 94720-4767 Search for other works by this author on: GSW Google Scholar Harold C. Helgeson Harold C. Helgeson 1Department of Geology and Geophysics, University of California, Berkeley, California 94720-4767 Search for other works by this author on: GSW Google Scholar Geology (1994) 22 (9): 851–854. https://doi.org/10.1130/0091-7613(1994)022<0851:SOPIOF>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Vitalii A. Pokrovskii, Harold C. Helgeson; Solubility of petroleum in oil-field waters as a function of the oxidation state of the system. Geology 1994;; 22 (9): 851–854. doi: https://doi.org/10.1130/0091-7613(1994)022<0851:SOPIOF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Aqueous solubilities of liquid nonane (C9H20 [liq]) with activities ranging from 1.0 in pure nonane to 0.1 and 0.01 in crude oil have been calculated at ∼120 °C and 400 bar as a function of oxygen fugacity (fO2(g), where g stands for gas) in a generic oil-field water containing representative concentrations of NaCl, CaCl2, MgCl2, and FeCl2 in equilibrium with minerals in the system CaO-MgO-FeO-CO2-O2-H2O. The calculations indicate that in the presence of calcite, the oxidative solubilities resulting from the formation of aqueous CO2, n-carboxylic acids, and other oxidized organic aqueous species from n-alkanes in crude oil are several orders of magnitude greater than their stoichiometric counterparts. For example, the oxidative solubility in the aqueous phase of nonane with an activity of 0.1 in crude oil coexisting with calcite at the oil-water interface increases dramatically with increasing log fO2,( g) at ∼120 °C and 400 bar from a stoichiometric solubility of ∼0.26 ppm to an oxidative solubility of ∼320 ppm at log fo2 (g)= -56, which is typical of the fugacity of oxygen in hydrocarbon reservoirs at ∼120 °C and 400 bar. Because oil-field waters are generally acidic, production of carbonic acid by the oxidative dissolution of crude oil in hydrocarbon reservoirs favors generation of secondary porosity at the oil-water interface, which may be mediated by hyperthermobarophilic microbes. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Grain size, composition, porosity and permeability contrasts within cross‐bedded sandstones in Tertiary fluvial deposits, central Spain

ABSTRACTPermeability measured with a portable probe permeameter on outcrops of cross‐bedded sandstones ranges between 0·9 and 19 D. The highest permeability (2–19 D with an average of 8·5 D) occurs in the coarsest grained foreset laminae (CFL), intermediate values (2–12 D with an average of 5·3 D) occur in finer grained foreset laminae (FFL) and the lowest values (0·9–10 D with an average of 4·8 D) occur in bottomset layers (BL). In the cross‐beds the average grain size ranges from medium grained sand in the CFL to fine grained sand in the FFL and BL. In all three subfacies, the average size of the primary pores is approximately 1φ unit smaller than the average grain size. The abundance of unstable carbonate clasts correlates with increasing average grain size, micritic clasts being most abundant in the CFL. Conversely, quartz content increases with decreasing grain size and is highest in the FFL and BL. Diagenetic destruction of primary porosity by compaction and cementation, as well as generation of secondary porosity through dissolution, were controlled by the original mineralogical composition of the sand. Contrasts in grain size determine the primary pore size contrasts and differences in composition between CFL, FFL and BL. Permeability contrasts reflect variations in average primary pore size rather than differences in total porosity. Probe permeability contrasts between CFL, FFL and BL depend on contrasts in average pore size and contrasts in mineralogical composition between the subfacies.

Influence of Provenance and Burial History on Diagenesis of Lower Cretaceous Frontier Formation Sandstones, Green River Basin, Wyoming

ABSTRACT The Upper Cretaceous Frontier Formation on the Moxa Arch in the western Green River Basin, Wyoming, has had a varied diagenetic history that was controlled in part by differences in composition of detrital framework grains and in burial history. Petrographic examination of 247 thin sections from 13 cores from the south-plunging arch and adjacent deep basin is the basis for diagenetic investigation of sandstones ranging in depth from 2 km to almost 5 km. Major diagenetic events were (1) mechanical compaction by grain rearrangement and deformation of ductile grains, (2) formation of illite and mixed-layer illite-smectite rims, (3) precipitation of quartz overgrowths, (4) precipitation of calcite cement, (5) generation of secondary porosity by dissolution of feldspar, chert, biotite, and mudstone grains and calcite cement, (6) precipitation of kaolinite in primary and secondary pores, and (7) chemical compaction by intergranular pressure solution and stylolitization and additional precipitation of quartz cement. The northern and southern ends of the Moxa Arch differ in the magnitude of each of these diagenetic events. Provenance differences caused more abundant ductile rock fragments and feldspar to be deposited at the northern end of the Moxa Arch. As a result, Frontier sandstones from the northern Moxa Arch underwent more extensive mechanical compaction. In addition, feldspar dissolution and albitization buffered acid-rich basinal fluids at the northern end, resulting in greater development of secondary porosity and precipitation of calcite cement than at the southern end. Deeply buried sandstones at the southern end of the arch and in the basin contain the most abundant quartz cement because intergranular pressure solution and stylolitization liberated silica for overgrowths.

Dolomite‐rock textures and secondary porosity development in Ellenburger Group carbonates (Lower Ordovician), west Texas and southeastern New Mexico

ABSTRACTPervasive early‐ to late‐stage dolomitization of Lower Ordovician Ellenburger Group carbonates in the deep Permian Basin of west Texas and southeastern New Mexico is recorded in core samples having present‐day burial depths of 1.5–7.0 km. Seven dolomite‐rock textures are recognized and classified according to crystal‐size distribution and crystal‐boundary shape. Unimodal and polymodal planar‐s (subhedral) mosaic dolomite is the most widespread type, and it replaced allochems and matrix or occurs as void‐filling cement. Planar‐e (euhedral) dolomite crystals line pore spaces and/or fractures, or form mosaics of medium to coarse euhedral crystals. This kind of occurrence relates to significant intercrystalline porosity. Non‐planar‐a (anhedral) dolomite replaced a precursor limestone/dolostone only in zones that are characterized by original high porosity and permeability. Non‐planar dolomite cement (saddle dolomite) is the latest generation and is responsible for occlusion of fractures and pore space.Dolomitization is closely associated with the development of secondary porosity; dolomitization pre‐and post‐dates dissolution and corrosion and no secondary porosity generation is present in the associated limestones. The most common porosity types are non‐fabric selective moldic and vuggy porosity and intercrystalline porosity. Up to 12% effective porosity is recorded in the deep (6477 m) Delaware basin. These porous zones are characterized by late‐diagenetic coarse‐crystalline dolomite, whereas the non‐porous intervals are composed of dense mosaics of early‐diagenetic dolomites. The distribution of dolomite rock textures indicates that porous zones were preserved as limestone until late in the diagenetic history, and were then subjected to late‐stage dolomitization in a deep burial environment, resulting in coarse‐crystalline porous dolomites. In addition to karst horizons at the top of the Ellenburger Group, exploration for Ellenburger Group reservoirs should consider the presence of such porous zones within other Ellenburger Group dolomites.

Depositional and Diagenetic Controls on Gas Production from Frio (Oligocene) Sands at Monte Christo Field, Hidalgo County, Texas

ABSTRACT Monte Christo Field is located in central Hidalgo County on the Gulf Coastal Plain of south Texas. Depositional and structural factors controlling the accumulation and production of hydrocarbons are related to lateral and vertical facies changes as well as syndepositional growth faulting. These factors combine to compartmentalize the pay sandstones into discrete, isolated reservoirs. Rollover into these growth faults, coupled with greater sandstone thickness and quality at preferred points of delta-front sand deposition, result in a combination of structural and stratigraphic entrapment of gas. Frio reservoir sandstones consist of stacked, upward-coarsening, progradational packages. In each is found a vertical succession of basal abandonment facies, offshore mudstones, distal-delta front sands and silts (lower shoreface), and distributary mouth bar (upper shoreface) sandstones. The upper shoreface sandstones comprise the best reservoir facies. Each facies contains trace fossils indicative of the paleo-environment. Cyclic progradational packages at Monte Christo Field are the direct result of episodic movement along the Mission fault, a large down-to-basin growth fault which bounds the field to the west and parallels depositional strike. Within the study area, three smaller splinter faults bifurcate to the east from the Mission fault and compartmentalize the Frio reservoirs. Diagenetic influences on reservoir quality involve extensive cementation and secondary porosity generation. Reservoir sandstones are fine- and very fine-grained feldspathic litharenites and lithic arkoses. The breakdown of unstable volcanic rock fragments results in significant chlorite and illite grain-coating cements which commonly block pore throats and greatly reduce permeability. Calcite and ferroan carbonate cementation occludes porosity significantly. Minor quartz, potassium feldspar, zeolite and titanium oxide authigenesis are also present. These factors result in relatively low porosities of 10 to 15% and very low permeabilities. The generation of secondary porosity by dissolution of feldspars and carbonate material enhances the diminished pore volume. Point counts reveal that a minimum of 30% of the existing porosity is of secondary origin. Even with significant porosity enhancement in some intervals, artificial fracture stimulation is required to achieve an economic rate of production. This procedure also taps into the substantial quantities of gas trapped in the lowest permeability portions of the reservoir facies.

Role of Secondary Porosity and Permeability in Predrill Prediction of Total Porosity and Permeability in Sandstones

The subjective nature of some petrographic criteria used to define and quantify secondary porosity and the inadequacy of the mechanisms invoked to explain its generation make it difficult to construct realistic retrospective models of secondary porosity generation. Without such models, secondary porosity prediction, separate from total porosity prediction, is reduced to the realm of speculation. Fortunately, data from a number of sandstone reservoirs and targets suggest that empirical predictions of total porosity are possible even if secondary enhanced porosity is volumetrically important. This is particularly true of sandstones in which secondary porosity is formed predominantly by dissolution of framework grains. Empirical predictions are based on the correlation between porosity and permeability (dependent variables) and a limited number of parameters (independent variables) obtained from calibration data sets or estimated from appropriate geological models. It appears that accurate empirical predictions of reservoir quality in many sandstones affected by leaching are possible because the extent of secondary porosity and permeability generation is to a large extent a function of primary porosity and permeability. Whereas the empirical approach appears to be successful in predicting reservoir quality in sandstones affected by leaching of framework grains, its predictive accuracy in sandstones whose reservoir quality is controlledmore » by dissolution of authigenic cements remains to be determined. However, in many reservoirs the effect of dissolution of authigenic cements on reservoir quality may have been greatly overestimated.« less

EXPERIMENTALLY‐SIMULATED STYLOLITIC POROSITY IN CARBONATE ROCKS

An experimental program of stylolitic porosity development in stylolitic Atokan (Middle Pennsylvanian) limestones under simulated conditions of deep burial has shown that experimental dissolution preferentially occured and developed along existing styloties. The dissolution sequence was as follows: incipient pores following the traces of stylolites, grading into stylolitic lamellar pores, which enlarged eventually into non‐fabric‐selective channels. These stylolite‐controlled conduits were surrounded by adjacent incipient‐to‐complete oomolds, enlarged oomolds, incipient‐to‐complete algal bioclast molds, enlarged biomolds, and halos of intercrystalline porosity to microporosity.Trace element analysis (Mg, Sr, and Ca) of pore‐fluid samples during early phases of experimental deep burial dissolution indicated that slightly less stable protions of carbonates enriched in Mg and Sr dissolved preferentially, as observed also in natural cases. Pyrolysis data indicate that stylolites concerntrated organic matter derived from inter‐stylolite areas. This situation demonstrates the pressure‐solution origin of stylolites.Experimental stylolitic pore networks are similar in type, size, and geometry to natural stylolitic porosity. This preferential dissolution suggests that natural stylolites result from late (post pressure‐solution dissolution, are not necessarily mechanically induced during stylolite formation as sometimes postulated, and act as effective conduits for secondary porosity generation in deeply‐buried carbonate rocks

Relationship Between Diagenesis and Reservoir Quality in Sandstones of the Garn Formation, Haltenbanken, Mid-Norwegian Continental Shelf(1)

A quantitative model is proposed for the dependence of porosity and permeability on diagenesis in subarkosic arenites of the Middle Jurassic Garn Formation. Porosity variation is controlled by the combined effects of compaction and quartz cementation. In general, more porosity has been lost by compaction than by quartz cementation, but compactional porosity loss appears to have occurred relatively early in the burial history (before burial below 2 km below the sea floor). Thus, progressive reduction of the total porosity with increasing burial below 2 km results mainly from quartz cementation. Permeability is a function of the abundance of intergranular macroporosity, which is interpreted to be a measure of porosity. At depths less than 3.5 km below the sea fl or, intergranular macroporosity generally comprises more than 50 percent of the total porosity, but at greater depth, as total porosity decreases below about 16 percent, the ratio of effective to ineffective porosity drops below one. This depth corresponds to the onset of extensive illitization and grain dissolution, which results in major reorganization of the pore system. Generation of secondary porosity by feldspar dissolution appears not to have been beneficial to reservoir quality in the Garn Formation, but it is regarded as an intrinsic part of the overall process of porosity destruction with increasing diagenesis.

Timing diagenesis in the Tartan Reservoir (UK North Sea): constraints from combined cathodoluminescence microscopy and fluid inclusion studies

Fluid inclusions occur in authigenic quartz, baryte, ankerite and calcite cements that partially occlude inferred secondary porosity in Piper Formation sandstones of the Tartan field and adjacent Witch Ground Graben. Detailed petrography combined with hot cathodoluminescence studies enables a fluid inclusion stratigraphy to be related to the authigenic minerals and individual cement zones within them. Microthermometric studies indicate that these inclusions formed at elevated temperatures in the range 70–120°C. These temperatures are interpreted to indicate hot migrating fluids which invaded the on-structure sandstones at burial depths exceeding 1.5 km during the late Cretaceous-early Tertiary. Between successive cement generations major fluctuations in salinity are recorded. There is a marked lowering of salinity between subsequent quartz generations. Baryte and calcite cements, associated with metal sulphides, were precipitated from a low salinity brine. Ankerite cement at the present oil-water contact records precipitation from high salinity pore fluids. The salinity variations are thought to reflect large scale sub-surface fluid migration. Sources of acidity for the generation of secondary porosity and sources of silica, sulphate and base metals for the pervasive late cements cannot all be found within the reservoir sequence. This therefore indicates mass transfer within an open system. The complex diagenetic assemblage is unlikely to have been precipitated from the evolution of a single pore fluid. Mixing of at least two sub-surface fluids of very different chemistry and origin is inferred. Sulphate for barytes and sulphide cements is thought to have been derived as a result of cross-formational flow from Zechstein anhydrite juxtaposed against the reservoir prior to oil migration. Homogenization temperatures of fluid inclusions within cements along major fault planes are offset towards significantly higher temperatures than those inclusions distanced from such faults. This, together with the distribution of porosity and cements on the crestal structures and adjacent to major faults, strongly suggests that the faults were the condults for migrating hot fluids which mixed with the sulphate brine on the Tartan structure. Seismic valving provides an elegant mechanism for transporting large volumes of hot fluids intermittently but repeatedly along the major bounding faults. The source of the hot fluid is inferred to be related to the dehydration and illitization of smectite reaction, maturation of organic matter and expulsion of pore waters in the adjacent Witch Ground Graben, all of which took place broadly coincident with the generation of secondary porosity and late on-structure cementation.

Diagenetic Evolution of Cretaceous-Tertiary Turbidite Reservoirs, Campos Basin, Brazil

Three sandstone turbidite sequences of the Campos basin (offshore, state of Rio de Janeiro, Brazil) were petrologically studied: (1) the Albian-Cenomanian Namorado Sandstone on the Macae Formation, (2) the Upper Cretaceous Carapebus Member of the Campos Formation, and (3) the Eocene Carapebus Member of the Campos Formation. The sequences represent deep marine deposits consisting mostly of massive sandstones rather than classical turbidites, indicating sand-rich submarine fans were the main depositional system of these sequences. The framework composition of the sandstones averages for quartz, feldspar, and lithics are Q60F40Ltr for the Cretaceous rocks and Q71F29Ltr for the Eocene rocks, plotting granitic r ck fragments at the feldspar pole. The main diagenetic phases that affected the sandstones studied were (1) development of a clay matrix due to compaction of rip-up mud clasts, (2) partial replacement of the matrix by opal, (3) precipitation of small pyrite framboids, (4) widespread direct precipitation or replacement of different materials by calcite, (5) intense generation of secondary porosity, (6) localized kaolinite development, (7) minor precipitation of quartz and feldspar overgrowths, (8) development of dolomite, ferroan dolomite, and ankerite, and (9) replacement of different materials with minor direct precipitation of late pyrite. Geologic and geochemical evidence lets us infer the main processes that controlled diagenetic transformations and mass transfer within the sequence studied. The principal source of carbonate cements was pressure solution of the underlying Macae Formation. Most of the diagenetic evolution of the sandstones was apparently controlled by the relative balance between the activity of CO2 and carboxylic acid species in the formation waters, both related to organic matter transformations within adjacent marine shales. The timing of hydrocarbon (generated in deeper lacustrine shales) migration to the reservoirs was an important factor in preserving diagenetically enhanced porosity.

EVOLUÇÃO DIAGÉNETICA DOS TURBIDITOS DA FORMAÇÃO BOM GOSTO NA ÁREA LESTE DA BACIA DE BARREIRINHAS, MA

The Albian BoM Gosto Formation wits deposited in a marine environment during the initial phase of the Barreirinhas Basin formation, in the equatorial Brazilian margin. Arkoses are the main type of sandstones, varying from fine to coarse grained and showing a moderate sorting. These sediments have been subjected to a complex sequence of diagenetic events controlled by basin tectonics and provenance. The main phenomena are the results of chemical reactions in the mesodiagenetic stage. The chronologic sequence of diagenetic processes consists of: 1. mechanical compaction; 2. quartz and feldspar overgrowth; 3. non- ferroan calcite and Fe-calcite cementation and replacement followed by partial dolomitization ; 4. development of secondary porosity essentially by the dissolution of calcite replacement and cement; and 5. porosity reduction by mechanical compaction and precipitation of clay minerals (mainly kaolinite), quartz, feldspar, pyrite and anatase into the pore system, The secondary porosity was classified in four main categories: A, obvious dissolution porosity, subdivided into six classes based on corrosive and/or geometric grain features; B, regular intergranular porosity; C, microporosity, divided into two classes based on the textural position of kaolinite within the framework; and D. fracture porosity. Correlation between porosity classes and depth shows the importance of the ascendant leaching fluids which are responsible for most part of the secondary porosity generation, as well as for the transport of aluminium and silicium, which precipitated as kaolinite under favourable conditions. Silica probably was derived by pressure solution of quartz grains and feldspar replacement by calcite and kaolinite. Carbonate cement probably has its origin related to diagenetic phenomena within the adjacent shales (Arpoador and Tutoia formations). Authigenic clay minerals apparently have been formed through reactions which involved the dehydration of shales. They are the main agents of the low permo-porosity observed in Born Gosto sandstones, once they reduce the secondary porosity and modify the pore system through particle migration and create large amounts of microporosity.

Depositional and diagenetic factors in the generation of the Santiago arenite reservoirs (lower cretaceous): Araçás oil field, Recôncavo Basin, Brazil

The Santiago Member of the Pojuca Formation in the Araçás oil field consists of deltaic sandstone bodies interbedded with shales which were deposited in an interdistributary bay area through breaching of channel levees. The best reservoirs occur in crevasse mouth bar deposits and consist of quartzose feldspathic arenites and feldspathic arenites. The diagenetic evolution of the reservoir arenites consists of: destruction of primary porosity by compaction and burial calcite cementation; development of 35% secondary porosity by leaching of calcite cement; decrease in secondary porosity to 20% by late cementation (kaolinite, chlorite, pyrite); and preservation of secondary porosity by oil migration which halted further diagenesis. Framework grains display distribution patterns controlled by features of the depositional environments. Diagenetic processes in pure arenites, controlled by a dome-like compaction structure, directed the dissolution of cement and the generation of secondary porosity. Both cement and secondary porosity are concentrated in the dome center and display closed area patterns of distribution in which values vary in opposition to each other. Intercalation of matrix-rich microfacies in any reservoirs inhibits dissolution. This leads to a reverse situation in which porosity still displays a closed areal pattern of distribution but concentrates in the dome periphery. In all reservoirs, permiability distribution follows neither the distribution of framework grains, nor of cement, nor of porosity, indicating that it results from a complex and as yet not understood interaction of factors.

Diagenesis of Tertiary playa sandstones of Northern Chile; implications for Andean uplift and metallogeny

ABSTRACTThe Oligo‐Miocene Pacencia Group of Antofagasta Province, northern Chile consists of over 2 km of molassic sediments deposited in a series of alluvial fan and playa sub‐environments. Sandstones of the Artolla Member, exposed around San Bartolo, represent playa‐marginal sandflat deposits and locally host stratiform copper mineralization. A detailed study of these sediments has revealed a complex sequence of diagenetic modifications to the arkosic detrital mineral assemblage. Early dissolution of unstable ferro‐magnesian minerals and calcic feldspar resulted in the release of ions into intra‐stratal solution and the subsequent formation of a suite of eogenetic authigenic minerals.These include early hematite coatings on framework grains, pore‐lining zeolite and pore‐filling calcite, gypsum and celestite cements. Syntaxial overgrowths of albite, quartz and less common K‐feldspar are well developed. Following early oxidative intrastratal conditions, the influx of acidic fluids derived from mudrocks during mesogenesis resulted in an important secondary porosity generation and the establishment of an acidic, reducing intrastratal environment. This facilitated the subsequent introduction of low temperature mineralizing solutions and formation of the San Bartolo copper deposit.A return to oxidative diagenetic conditions is recorded by late stage overgrowths of K‐feldspar and quartz and the oxidation of native copper/chalcopyrite cements to cuprite/tenorite and chalcocite/covellite assemblages. This final telogenetic modification to the mineral assemblage is related to flushing by meteoric waters and provides independent evidence for an important phase of Andean deformation and uplift in the middle to upper Miocene.

Sandstone Petrology and Diagenesis of Lower Tuscaloosa Formation Reservoirs in McComb and Little Creek Field Areas, Southwest Mississippi: ABSTRACT

ABSTRACT Two major depositional facies characterize producing Lower Tuscaloosa Formation (Upper Cretaceous) sandstone reservoirs in the McComb-Little Creek Field areas; a lower fluvial sequence (Denkman Sand, Little Creek) topped by near shore marine deposits (McComb Sand). Petrographic studies reveal that the sandstones of both fields were deposited as very fine to medium-grained quartz arenites and quartz litharenites. Petrographic and X-ray diffraction analysis of the clay mineral content of the sandstones indicate that most of the clays are authigenic and comprise a suite which includes chlorite, kaolinite, mixed-layer illite/smectite and illite. Mixed-layer illite/smectite appears to be considerably more abundant in the McComb reservoir sandstones than at Little Creek. The reservoir sandstones in the McComb and Little Creek Fields are highlighted by good secondary porosity (average 19%, range 32 to 27%) developed through dissolution of rock fragments and carbonate cements. The combined petrographic and X-ray diffraction results suggest a diagenetic model which begins with mechanical compaction of the sediments and the precipitation of quartz overgrowths. Carbonate cements replaced quartz overgrowths and filled interstices between framework grains during the first stages of mesodiagenesis. During a more mature diagenetic stage, perhaps corresponding to hydrocarbon migration and emplacement, decarboxylation of contained organic matter in interbedded and adjacent fine-grained units led to wide-spread decarbonization of the reservoir sandstones and the creation of hybrid, oversized, moldic and intergranular pores with good pore-throat interconnection. Complete and partial alteration of rock fragments and feldspars was accompanied by neoformation of chlorite (as grain coatings), kaolinite, and illite. The formation of vermicular kaolinite occurred after the main phase of secondary porosity generation and hydrocarbon emplacement. Late stage diagenetic events also include the precipitation of euhedral quartz crystals in some pore spaces and over chlorite-coated grains.

The development and destruction of porosity within Upper Jurassic reservoir sandstones of the Piper and Tartan Fields, Outer Moray Firth, North Sea

AbstractUpper Jurassic Piper Formation sandstones of the Outer Moray Firth Basin, UK North Sea, form the main reservoir for the Piper and Tartan Fields, and are sealed by either the Kimmeridge Clay or Cretaceous marls. The main reservoir sandstones can be broadly considered as stacked coarsening-upward units that accumulated in a predominantly high-energy, shallow marine complex with depositional waters of inferred normal marine salinity, Eh and ionic composition. The amount of porosity and its distribution throughout these sandstones differs between the individual structural entities of the Piper and Tartan Fields. On the Piper structure all the sandstones are highly porous. Oil-zone porosities average 25% and are only slightly reduced in the water zone. Sandstones in the upthrown block of the Tartan structure are also generally porous but porosities are typically reduced to below 20%. Porosities are the most variable in the Tartan downthrown block and in the 15/17-9 well, off-structure Piper, where porous sandstones with between 14 and 18% porosity are associated with highly compacted and cemented sandstones with &lt;5% porosity. Thin-section and SEM textural evidence indicate that much of the porosity in these sandstones is secondary, produced through the dissolution of an intergranular cement that provided support for the detrital framework, was peripherally replacive with respect to most detrital grains, and extensively replaced feldspars. Nodular concretions and sporadic, irregular crystals of ferroan calcite are inferred to be the remnants of this intergranular cement. The distribution of porosity zones in the Tartan downthrown block is related to the present structure and oil-water contact, supporting the interpretation of a secondary origin for much of the present porosity. Highly compacted sandstones interbedded with the porous sandstones result from a combination of early compaction, brittle framework collapse following the generation of secondary porosity and late grain-to-grain contact dissolution. Aggressive fluids responsible for the generation of the cement dissolution porosity are inferred to have been expelled from the Kimmeridge Clay in the adjacent Witch Ground Graben. The preservation of secondary porosity in Piper Formation sandstones is due to the relative timing of secondary porosity generation and subsequent hydrocarbon migration.

Dissolution of apatite in North Sea Jurassic sandstones: implications for the generation of secondary porosity

AbstractHeavy-mineral studies on Jurassic sandstones from the central and northern North Sea areas and from the Lossiemouth Borehole (onshore NE Scotland) show that the dissolution of apatite is a function of depositional environment rather than burial depth. In the shallow marine Upper Jurassic sands of the Claymore, Clyde and Tartan Fields, and in the deeper-water Magnus sands, apatite is ubiquitous, even where burial depths exceed 3800 m. Conversely, the fluvio-deltaic sands of the Beatrice, Heather, Ninian and Murchison Fields, and of the Lossiemouth Borehole, have suffered apatite dissolution, although burial depths range from very shallow (Lossiemouth Borehole) to about 3300 m. This clearly indicates that apatite dissolution has taken place through penetration of low-pH meteoric groundwaters at a very early stage in diagenesis, and that high-temperature fluids circulating in deep burial have had little or no effect. This is in accord with patterns of mineral dissolution observed in other sedimentary basins and in the North Sea Palaeocene. Although dissolution of heavy minerals is unlikely to generate significant secondary porosity, the process is nevertheless caused by the same pore-fluids that dissolve major framework constituents. Patterns of heavy-mineral dissolution therefore provide clues to the nature of these pore-fluids. Here, the relative stability of apatite is particularly significant. The order of stability apatite &gt; garnet &gt; kyanite, which characterizes deep burial of North Sea sandstones, has previously been simulated experimentally using fluids of pH 8 at room temperature. This suggests that high-temperature acidic pore-fluids may not have played a significant role in the development of secondary porosity in North Sea sandstones.

Diagenetic history of Eocene Wilcox sandstones, South-Central Texas

Sandstones and shales of the lower Eocene Wilcox Group extend from outcrop to depths greater than 5 km where temperatures exceed 200°C. Eight authigenic minerals have been identified. Quartz, kaolinite, calcite, ankerite and albitized plagioclase are volumetrically significant, whereas chlorite, illite, albite overgrowths and dolomite are present only in trace amounts. Quartz, kaolinite, and ankerite cements are present over the entire depth range sampled (1528 to 4446 m). Calcite cement occurs only in sandstones shallower than 3300 m. Albitized plagioclase is the predominant feldspar in samples deeper than 2600 m whereas detrital potassium feldspar has been completely dissolved out of sandstones deeper than about 3200 m. No systematic variations in the chemical composition of calcite with depth, no zoning within calcite or ankerite, and no compositional differences between pore-filling and grain-replacing calcite or ankerite were found. Calcite cements have 87Sr 86Sr ratios generally less than 0.708, whereas ankerite cements are significantly enriched in 87Sr. The oxygen isotopic composition of carbonate cement ranges between −9 and −14%. (PDB) for calcite and between −8 and −12%. for ankerite with no systematic variation with depth. Calculated δ 18O values for authigenic quartz and kaolinite range from +22 to +31%. (SMOW) and from +14 to +17%., respectively. Volumetrically important amounts of authigenic quartz and kaolinite precipitated during early burial before calcite cementation, secondary porosity generation, or smectite to illite transformation in encasing shales. The isotopic composition of strontium in calcite cement shows most of it to be of Eocene or older marine carbonate origin. Ankerite cementation followed the generation of secondary porosity and occurred after the products of organic maturation and shale diagenesis, plagioclase albitization or potassium feldspar dissolution significantly affected the sandstone-pore water system.

Deformation in Thrust-Ramp Anticlines and Duplexes: Implications for Geometry and Porosity: ABSTRACT

A computerized kinematic model of thrust-ramp anticline geometry allows workers to predict the zones of greatest deformation in ramp anticlines and fault duplexes. The model assumes a constant cross-section area, symmetrical fold hinges, and slip in the hanging wall parallel to the ramp and forelimb. Assuming that the collapse of original porosity or the generation of secondary fracture porosity is proportional to deformation, the model can be used to predict porosity changes. Deformation in a single ramp anticline is greatest in the forelimb and backlimb, and may be absent in the crest. A duplex structure results from comparatively closely spaced thrusts that have a common upper detachment horizon. Relatively wide spacing between the duplex faults yields a bumpy roofed duplex as in the central Appalachians. Forelimbs may be deformed twice and should show greater porosity modification. Relatively close spacing between ramp-and-flat thrusts can produce a listric-fault, snakehead anticline geometry because younger faults deform the preexisting thrust slices. The resulting geometry is here called a snakehead duplex and appears to be fairly common, as in the Jumpingpound field in the Canadian Rockies. Each thrust slice within the duplex is deformed six times or more, providing the maximum opportunity for deformation-related porositymore » changes. Maximum fracture porosity should occur in thrusts having listric-fan or snakehead duplex geometry. Structures involving duplexes generally should be better than isolated ramp anticlines.« less

Porosity Prediction in Quartz-Rich Sandstones: Middle Jurassic, Haltenbanken Area, Offshore Central Norway: ABSTRACT

Reduction of primary porosity by mechanical compaction and pressure solution is the main control of reservoir quality in the upper member of Middle Jurassic, quartz-rich (Q > 75%) sandstones in the Haltenbanken area. The extent of secondary porosity and permeability generation is limited to volumetrically minor alteration or dissolution of feldspars. The effects of pressure solution, exhibited by petrographic textures, vary widely. The extent of significant porosity reduction by pressure solution was determined to be a function of the residence time of the sandstones within the pressure-solution domain, a region roughly below 1500-m burial depth in the Haltenbanken area. Porosity prediction curves were developed from plots of measured porosity and an integration of the burial history within the pressure-solution domain (i.e., the product of burial time and depth between 1500 m and present-day depth). The curves, both for compacted and decompacted profiles, were constructed using data from six wells in the Haltenbanken area. Porosities of three wells drilled after the porosity prediction approach was developed, closely correspond to the predicted porosity values. Average permeability also can be predicted relatively accurately because of the good correlation between porosity and permeability in these quartz-rich sandstones. The porosity-burial history relationship can be applied more » to porosity predictions prior to drilling in the Haltenbanken and related areas using burial histories interpreted from seismic-stratigraphic analysis. « less