Secondary Porosity In Sandstones Research Articles

Modelling diagenetic reactions and secondary porosity generation in sandstones controlled by the advection of low‐molecular‐weight organic acids

AbstractHigher secondary porosity was observed in the centre of a sandstone unit in the Eocene Shahejie Formation fan delta front sandstones from the Bozhong Depression, Bohai Bay Basin. This differs from past studies showing secondary porosity mainly in the marginal parts of sandstones adjacent to shales. This study utilized reactive transport models involving low‐molecular‐weight organic acids (LMWOA) to discuss potential processes resulting in the contrary distribution of secondary porosity. An interface model simulating LMWOA diffusion from adjacent shales to the sandstone resulted in secondary porosity in sandstones adjacent to shales. In contrast, an advection model simulating advective transport of LMWOA parallel to the sandstone bedding successfully generated higher secondary porosity in the central part. The central part of the sandstone exhibited better grain sorting (greater depositional porosity) and significantly less early carbonate cements compared to the marginal sandstone parts. Consequently, the central part had greater porosity prior to the dissolution through LMWOA. The initially higher porosity in the central part allowed for a higher advective flux of LMWOA‐rich water and associated lower pH, resulting in decreased oligoclase saturation, higher oligoclase dissolution rates and ultimately higher secondary porosity. This study indicates that grain sorting during sediment deposition, early carbonate cementation, LMWOA production in adjacent shales, and advection processes collectively control the diagenetic reactions and the distribution of secondary porosity in sandstones.

DISTRIBUTION OF CARBONATE CEMENTS IN THE SANDSTONES OF THE SOUTHEAST OF THE DNIEPER-DONETSK DEPRESSION IN THE ASPECT OF FORMING SECONDARY POROSITY IN THEM AT HIGH DEPTHS

The main problem when searching for oil and gas at great depths is the lack of an economic reservoir. Since great depths correspond to high degrees of catagenetic transformations, the primary porosity of most rocks is reduced and all hopes are associated either with the relict primary porosity, the value of which progressively decreases with depth, or with secondary porosity. There are different views on the formation of secondary porosity in sandstones - the main reservoirs of the Dnieper-Donetsk Depression (DDD). Some of these views are briefly discussed in this paper. In all cases, carbonate cement plays an important role as its dissolution under the influence of various factors forms secondary porosity. The question arises as to how often sandstones with carbonate cement are found among the differences with a different cementing material, at what depths, in the sediments of what ages, are there many sandstones in which the formation of secondary porosity is possible by one or another mechanism. The aim of this work is to quantitatively assess the prevalence of carbonate sandstones among the sandy rocks of the southeastern part of the DDD, clarify the distribution of carbonate sandstones in depth and over the main stratigraphic horizons, and preliminary estimate the content of various carbonates in the cement of sandstones according to field and standard laboratory studies of core material. In the course of the work, data from more than 10 thousand samples were used, taken at various fields of the northern side and the northern near-zone zone of the southeastern part of the Far East region, approximately from the Belskoye field in the north-west to Kruzhilovskoye in the southeast inclusive. The sampling depth intervals for the samples used vary from 1 to 6 km, the age of the sediments they characterize is from Triassic to Devonian. The results obtained show that the content of carbonate in sandstone cement at great depths is high enough for its movement to cause the formation of secondary porosity on an industrially significant scale, which, in turn, provides good prospects for prospecting hydrocarbon deposits at great depths.

Comparative pore surface area in primary and secondary porosity in sandstones

Primary and secondary porosity in sandstones possess different pore geometry characteristics, but these are not well quantified. The pore surface area in 2 suites of sandstones exhibiting only primary porosity (Permo-Triassic, Northern Ireland) and only secondary porosity (Cambrian, England) were measured using JMicrovision software. The data show pore surface areas per unit pore volume ∼2.5 times as great in the secondary porosity compared to the primary porosity. This difference is great enough to have a significant impact on properties dependent on pore surface area, including oil production and capacity for microbial colonization in the deep biosphere.

The illitization of dickite: chemical and structural evolution of illite from diagenetic to metamorphic conditions

The illitization of dickite is commonly observed at diagenetic to low-grade metamorphic conditions in Triassic rocks from the Betic-Rif Cordillera. It has been studied by X-ray diffraction and by scanning and transmission/analytical electron microscopy. Dickite is a well-ordered diagenetic phase that occurs filling the secondary porosity in sandstones and conglomerates. In diagenetic samples, the alteration of dickite to illite produces thin 1 M mica crystals with incomplete interlayer occupancy (illite), wide chemical variability, and low Na contents. Under low anchizonal conditions, the illitization process is more advanced and 2 M 1 K-illite coexists with minor dickite, sudoite, and 1 M intermediate Na–K-illite. At higher anchizonal conditions only relics of dickite persist, whereas pyrophyllite coexists with 2 M 1 illite/muscovite with K-rich composition, indicating an approach to equilibrium. Alteration of dickite to illite and sudoite mainly progresses along the dickite layers and produces progressively larger packets of 1 M and 2 M 1 white mica. Alteration of dickite to pyrophyllite advances parallel and transverse to the dickite layers. The entry of appreciable amounts of Na into the illite structure seems to depend not on the rock composition but on the metamorphic conditions, i.e . temperature. The dickite to Na–K white mica transformation is an alternative, and probably common way, not previously described, for forming Na-bearing mica in coarse-grained smectite-poor rocks.

Burial diagenesis of illite/smectite in shales and the origins of authigenic quartz and secondary porosity in sandstones

Several gram-size samples of <0.1 μm, nearly monomineralic mixed-layer illite/smectite (I/S)were isolated from nineteen whole-cores of shale of the Frio Formation from the Texas Gulf Coast. Between ∼7,000 ft (2133 m) and ∼15,000 ft (4572 m) burial depth I/S changes from random-interstratification ( R = 0) with low %I to ordered-interstratification ( R = 3) with high %I. The accompanying chemical changes are loss of Si, Mg, and Fe +3, gain of K, Al, and Fe +2, and a shift in the REE content of the mineral. There is no chemical or mineralogical evidence for a cessation of I/S reaction at ∼80% I as is commonly accepted; both chemical and mineralogical changes occur throughout burial. Continuous change in the oxygen isotopic composition of I/S implies that the conversion of smectite-layers to illite-layers is a dissolution and reprecipitation process. Mass balance calculations indicate that burial diagenesis of Frio Formation shales is an open-system process that requires addition of K 2O and Al 2O 3 and results in loss of SiO 2. The amount of SiO 2 made available by shale diagenesis is sufficient to be the source of the quartz-overgrowth cements in the associated Frio sandstones. Iron reduction in I/S is a significant source of the acid required for the diagenesis of both Frio shales and sandstones.

Diagenetic Development of Clay and Related Minerals in Deep Water Sandstones (S. Spain): Evidence of Lithological Control

AbstractThe turbiditic facies of the Aquitanian-Oligocene of the Algeciras and Bolonia Units consist of lutitic-marly beds alternating with varied sandstones. Early diagenetic processes and the development of secondary porosity in sandstones were controlled largely by sedimentary facies. Mineral dissolution processes were important in rocks with high primary porosity (quartzose sandstones) and led to the development of allophane and kaolinite/dickite in later diagenetic stages. In rocks with high contents of biotitic matrix, chlorites and iron oxides were produced, together with the development of secondary porosity. In feldspathic sandstones, the early diagenetic processes produced chlorite, illite, illite-smectite mixed-layers and smaller amounts of kaolinite. In calcareous sandstones, the presence of an early cement prevented, to a certain extent, the dissolution of unstable fragments and restricted the development of clay minerals. The movement of fluids was not important during this early diagenetic stage in controlling the variety or abundance of mineral cements; the controlling factors were the different chemical conditions developed in the pore-solution of the various sandstone types. In spite of similar sandstone burial conditions, the different sandstone lithologies continued to determine the diagenetic processes, especially the degrees of compaction and dissolution. Certain burial processes are common to all levels: the development of reducing conditions is indicated by the development of Fe and Mn carbonates, Fe-dolomites and Fe-chlorites. A later alkaline diagenetic phase was responsible for the development of calcite, feldspar and illite. The circulation of fluids in these stages remained limited, although there was probably some movement from the lutitic-level fluids to the overlying sandstones.

Porosity development in deep sandstones, Taranaki Basin, New Zealand

Reservoir quality in the Taranaki Basin is controlled by both depositional facies and diagenesis. High initial porosities and permeabilities in manyfluvial and littoral sandstones directed fluid flow such that these facies underwater widespread cementation by silica, carbonate and authigenic clays, and later dissolution of carbonates. Current commercial production of hydrocarbons in mainly from secondary, dissolution porosity in sandstones at depths of 1750–3600 m. Recent studies suggest that oil may not be released from terrestrial source rocks in western New Zealand until depths of 5.5 km or more. Knowledge of porosity development at such depths is therefore essential for understanding migration pathways and the controls on hydrocarbon accumulation. Organic species and/or carbon dioxide evolved from kerogen before liquid hydrocarbons are important to the formation of secondary porosity in sandstones by carbonate dissolution. Further, hydrocarbon migration should follow soon after the evolution of these, in order that re-cementation does not occur. High resolution nuclear magnetic resonance (NMR) spectroscopy shows that in western New Zealand basins, the evolution of solvents continues to at least 4 km and may persist almost until the expulsion of liquid hydrocarbons.

Intergranular Volume (IGV) Decline Curves for Evaluating and Predicting Compaction and Porosity Loss in Sandstones

In contrast to conventional empirical porosity prediction schemes based on porosity decline with depth or thermal maturity, development of a conceptually sound porosity prediction algorithm requires quantitative assessment with depth (or thermal maturity) of two independent processes: compaction (as monitored by intergranular volume or IGV) and cementation. IGV is the sum of intergranular porosity, matrix, and cement as measured in thin section. For this study, in order to evaluate the effects on porosity of compaction alone, a plot of IGV versus depth was constructed using data from greater than 350 relatively uncemented ({lt}5% cement) sandstones from a wide variety of depths, ages, and geographic locations. Because texture and composition affect the initial IGV, the following additional restrictions were applied to the data set: {ge}0.1 mm grain size, well to moderately well sorted, {lt}5% matrix, {lt}15% feldspar grains, and {lt}5% ductile grains. The resulting IGV decline curve reveals that sands compact mechanically and IGV declines rapidly from 40 to 42% at the surface to about 28% at 5000 ft. Between 5000 and 8000 ft, decline continues slowly until the framework stabilizes at around 26%. No further significant IGV decrease is observed to the depth limits of the data set at 22,000more » ft. The most significant implications of this curve for porosity prediction in sandstones are that (1) primary porosity can be preserved to great depths in the absence of cement, suggesting that reported volumes of secondary porosity in sandstones are exaggerated, (2) intergranular pressure solution plays a minor role in localized quartz cementation, and (3) cement prediction is a major key to porosity prediction.« less

Secondary Porosity Revisited: The Chemistry of Feldspar Dissolution by Carboxylic Acids and Anions (1)

Carboxylic acids in subsurface waters have been proposed as agents for dissolving feldspars and complexing aluminum to create secondary porosity in sandstones. Previously published experimental work indicated high aluminum mobility in the presence of carboxylic acid solutions. In order to further evaluate aluminum mobility, alkali feldspar dissolution experiments were run at 100 degrees C and 300 bars in the presence of mono- and dicarboxylic acids and their anions. Experimental results imply that under reservoir conditions, aluminum-organic anion complexes are insignificant for acetate and propionate and possibly significant for oxalate and malonate. Propionate appeared to inhibit alkali feldspar dissolution and, hence, may retard aluminum mobility. Dissolution of feldsp r in the presence of oxalic and acetic acid can be explained by enhanced dissolution kinetics and greater aluminum mobility under low-pH conditions. The general absence of such low-pH fluids in subsurface reservoirs makes this an unlikely mechanism for creating secondary porosity. Also, the thermal instability of oxalate and malonate limits their aluminum-complexing potential in reservoirs at temperatures above 100 degrees C.

Ancient subaerial exposure and freshwater leaching in sandstones

Research Article| December 01, 1990 Ancient subaerial exposure and freshwater leaching in sandstones D. Emery; D. Emery 1BP Exploration Company, Ltd., Britannic House, Moor Lane, London EC2Y 9BU, England Search for other works by this author on: GSW Google Scholar K. J. Myers; K. J. Myers 1BP Exploration Company, Ltd., Britannic House, Moor Lane, London EC2Y 9BU, England Search for other works by this author on: GSW Google Scholar R. Young R. Young 1BP Exploration Company, Ltd., Britannic House, Moor Lane, London EC2Y 9BU, England Search for other works by this author on: GSW Google Scholar Author and Article Information D. Emery 1BP Exploration Company, Ltd., Britannic House, Moor Lane, London EC2Y 9BU, England K. J. Myers 1BP Exploration Company, Ltd., Britannic House, Moor Lane, London EC2Y 9BU, England R. Young 1BP Exploration Company, Ltd., Britannic House, Moor Lane, London EC2Y 9BU, England Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1990) 18 (12): 1178–1181. https://doi.org/10.1130/0091-7613(1990)018<1178:ASEAFL>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 D. Emery, K. J. Myers, R. Young; Ancient subaerial exposure and freshwater leaching in sandstones. Geology 1990;; 18 (12): 1178–1181. doi: https://doi.org/10.1130/0091-7613(1990)018<1178:ASEAFL>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 We present an example of leaching by meteoric water in the Jurassic Magnus Sandstone Member of the northern North Sea. The interpretation is based on information from several independent techniques, including downhole natural gamma-ray spectrometry, detailed petrographic analysis, and cross-sectional time-slice reconstructions from seismic data. The initial lines of evidence that prompted further investigation into freshwater involvement in the Magnus Sandstone Member were brackish formation-water compositions and a potassium depletion shown by the gamma-ray spectrometry log, in sandstone beneath the unconformity at the base of the Cretaceous section. Subsequent detailed petrographic study confirmed that the potassium depletion had been brought about by potassium feldspar dissolution, resulting in precipitation of kaolinite and creation of secondary porosity, the latter also revealed by conventional core-analysis porosity and permeability measurements. Finally, cross-sectional time-slice reconstructions from a regional seismic line across the Magnus oilfield strongly suggested subaerial exposure of the Magnus Sandstone Member in Early Cretaceous time and consequent freshwater leaching of the Magnus Sandstone Member and generation of the unconformity at the base of the Cretaceous. The combination of several lines of evidence from the Magnus field supports the process of subaerial exposure and freshwater leaching to create secondary porosity in sandstones. 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.

Secondary Porosity in Sandstones and Acid-Producing Reactions in Adjacent Shales, Oligocene, South Texas: ABSTRACT

Frio and Vicksburg sandstones in the McAllen Ranch and Hinde fields in south Texas show two depth-related zones of secondary porosity. Anomalously high porosity in the upper zone, from 4,000 to 6,500 ft (1,220 to 1,980 m), is related to the dissolution of analcime cement by acidic pore fluids released from adjacent shales. Much of this acid may be produced by the reduction of octahedral iron in smectite clay. Sodium released by analcime dissolution may be responsible for abnormally high salinities calculated for pore fluids in these sandstones. Calcite cementation below the upper zone may be related to fluids rich in calcium carbonate that have migrated up fault zones from great depth. The lower zone of secondary porosity extends from 8,400 to 10,000 ft (2,560 to 3,050 m) at Hinde and from 8,400 to 14,000 ft (2,560 to 4,270 m) at McAllen Ranch. The top of this zone also corresponds to the clay dehydration depth and the top of the abnormally pressured zone. At this depth and temperature, randomly interstratified illite and/or smectite clay becomes ordered and a large amount of water is expelled from the clay structure by potassium fixation. Smectite illitization produces minor amounts of kaolinite and chloritemore » and, more importantly, releases silica and acidic pore water. The acidic pore water migrates into adjacent sandstones and enhances the dissolution of feldspar grains and calcite, creating secondary porosity. The shales at McAllen Ranch have undergone more extensive illitization than those at Hinde, and more secondary porosity is observed in the McAllen Ranch sandstones. Data from detailed chemical and mineralogical analyses were used in mass-balance calculations to confirm that the amounts of acid produced by these reactions can account for the observed secondary porosity in these sandstones.« less

Indirect Evidence of Secondary Porosity in Sandstones: ABSTRACT

Indirect Evidence of Secondary Porosity in Sandstones: ABSTRACT

Formation of secondary porosity in sandstones by quartz framework grain dissolution

An understanding of the nature and origin of porosity in sandstones is crucial to the evaluation of their potential as hydrocarbon reservoirs. Primary porosity is normally destroyed by cementation, compaction and pressure solution during burial, but it has recently been recognized that significant secondary porosity can develop at depth1–3. This discovery has been called “the most significant advance in the study of clastic diagenesis in the past decade”4. Several genetic types of secondary porosity have been identified ; these include porosity formed by: (1) fracturing; (2) shrinkage; (3) dissolution of sedimentary grains and matrix; (4) dissolution of authigenic cements; and (5) dissolution of authigenic replacive minerals5. Formation of secondary porosity by framework grain dissolution has been thought mainly to involve decomposition of feldspars and lithic grains6,7.We demonstrate here that significant secondary porosity can also develop by dissolution of quartz framework grains if the rocks are exposed to alkaline pore fluids during deep burial and/or uplift.

Secondary Porosity in Sandstones: Basic Contributions of Chepikov and Savkevich

The concept of secondary porosity in sandstones--porosity caused primarily by dissolution--has gained considerable attention recently. The basic contributions made by Chepikov and Savkevich, however, have been ignored completely in North American publications. Chepikov et al not only developed the first set of criteria for recognizing secondary porosity, but also attributed the dissolution of cements to the arrival of oil. Savkevich was the first to illustrate the trend of increasing porosity at considerable depths.

Secondary Porosity in Sandstones: ABSTRACT

Secondary porosity in sandstones is caused by dissolution and fracturing, and is common in the sedimentary record. Secondary porosity commonly develops in the deep subsurface and thus provides an opportunity to extend exploration to depths traditionally considered unsuitable for exploration. Two contrasting routes of diagenesis exist in nature: porosity reduction and porosity enhancement. Porosity reduction is commonly caused by compaction and cementation, whereas porosity enhancement is primarily caused by dissolution of carbonate minerals. The dissolution of carbonate minerals in the deep subsurface can be attributed to release of carbon dioxide during thermal maturation of kerogen. Clay-carbonate reaction and flushing of undersaturated meteoric water from erosional unconformity are equally important mechanisms in generating secondary porosity at various depths. Two basic types of primary pores (intergranular and intragranular) and four basic types of secondary pores (grain fractures, rock fractures, intergranular, and intragranular) can be differentiated on the basis of (1) position of pores, (2) timing of origin, and (3) processes of origin. The proposed classification system is useful in inferring reservoir quality. Various types of secondary porosity are recognized using a comprehensive set of 20 criteria. The various criteria are based on manner of breakage, pore geometry, grain geometry, products of dissolution, and sediment packing. Examples of secondary porosity gathered in part from investigation of 2,000 thin sections will serve as a guide for recognizing secondary porosity in outcrop, in hand specimen, under the scanning electron microscope, and most importantly, under the petrographic microscope. New evidence suggests that silicate minerals, including quartz, dissolve more commonly than have been reported. The abundant occurrence of secondary porosity in reservoir sandstones emphasizes the importance of secondary porosity in evaluating deep reservoirs. Since the first recognition of secondary porosity by Nutting in 1934, Chepikov in 1959 and 1961 not only developed the first set of criteria for recognition of secondary porosity but also introduced the concept in which development of secondary porosity is related to arrival of oil. End_of_Article - Last_Page 547------------

Role of Organic and Inorganic Reactions in Development of Secondary Porosity in Sandstones: ABSTRACT

The development of secondary porosity in many sandstones is the result of aluminosilicate and/or carbonate dissolution. The dissolution of aluminosilicates and creation of secondary porosity is a problem of aluminum mobility. Our experimental data demonstrate that it is possible to increase significantly the mobility of aluminum and to transport it as an organic complex in aliphatic acid solutions. The same aliphatic acid solutions have the capability of destroying carbonate cements and debris. Carothers and Kaharka have shown that concentrations of aliphatic acid anions range up to 5,000 ppm over the temperature range 80 to 200°C in some oil field formation waters. Our experiments show that acetic acid solutions at the same concentrations and over the same temperature range can increase the solubility of aluminum by a half an order of magnitude, whereas oxalic acid solutions increase the solubility of aluminum by an order of magnitude. The textural relations observed in the experiments are identical to those observed in sandstones containing secondary porosity as a result of aluminosilicate dissolution. A natural consequence of the burial of sedimentary prisms is the maturation of organic material. These maturation reactions result in the evolution of significant amounts of organic acids and carbon dioxide. The experiments suggest that the development of secondary porosity in a sandstone as a result of aluminosilicate or carbonate dissolution is the natural consequence of the interaction of organic and inorganic reactions during progressive diagenesis. The degree to which secondary porosity develops depends on the ratio of organic to inorganic matter, the sequence, rates and magnitude of diagenetic reactions, fluid flux, and sand/shale geometry. End_of_Article - Last_Page 635------------

Formation of Secondary Porosity in Sandstones: How Important is It and What are Controlling Factors?: ABSTRACT

Secondary porosity in sandstones may form by chemical dissolution of grains or cements. Care must be taken to distinguish between local dissolution and reprecipitation which do not increase the net porosity, and leaching which produces a net increase in the porosity, requiring a throughflow of large volumes of undersaturated pore water to remove the reaction products in solution. Such undersaturated pore water may be derived from: (1) meteoric water driven by a hydrostatic head; (2) subsurface pore water made acid by the release of CO2 from maturing kerogen; and (3) clay mineral reactions involving transformation of kaolinite and smectite to illite and dissolution of feldspar and carbonate--(Al2Si2O5(OH)4 + KAlSiO8 = KAl3Si3O10(OH)2 + 2SiO2 + H2O). In the Jurassic sandstones of the North Sea (Statfjord field), secondary porosity is formed during early diagenesis from meteoric water dissolving feldspar. However, the kaolinite formed as a product of the leaching may form aggregates of pore-filling cement, reducing porosity and especially permeability. Petrographic examination and microprobe analyses show that clastic feldspar is rimmed by authigenic feldspar indicating that leaching was not important in creating secondary porosity End_Page 549------------------------------ during deeper burial. Calculations of the total amounts of CO2 released by kerogen in sedimentary basins suggest that secondary porosity is favored by high contents of humic kerogen, but that the CO2 produced and/or pore-water flow in many types of basins are too small to create significant amounts of secondary porosity. Reactions with clay minerals may, however, provide additional sources of acid. End_of_Article - Last_Page 550------------

Mechanism for Framework Grain Dissolution (Secondary Porosity in Sandstones): ABSTRACT

We propose that organic and clay maturation in concert are responsible for much framework grain dissolution (secondary porosity). Petrographic observations indicate a pulse of porosity formation near the top of the oil-generation window and that there is often not enough authigenic clay to account for the aluminum removed from the dissolved grains. Geochemical considerations indicate that H+ ions are required for aluminosilicate dissolution and that the aluminum must be complexed to concentrations greater than 100 ppm in order to transport aluminum out of the sandstone using water volumes available in most basins. The smectite to illite conversion, which is coincident with early organic maturation, produces additional pore water and can desorb organic molecules from the s ectite interlayers. The early stages of organic-matter maturation generates H+ (as carbon dioxide), volume-change pressures to move fluids, and water-soluble organic matter. The soluble organic matter can contain ligand compounds (e.g., short-chain fatty acids) which complex aluminum. The organic ligands in the shale complex aluminum at relatively low concentrations because aqueous aluminum activity is depressed by the formation of illite from smectite. The H+ and organic ligand-bearing solution is expelled into sandstones where the aluminum activity is buffered at higher levels by feldspar, thus allowing higher levels of complexed aluminum. The solution dissolves the feldspars and other aluminosilicate components and complexes much of the resulting aluminum for transport out of the sand tone. End_of_Article - Last_Page 630------------

Migration of Hydrocarbons and Secondary Porosity in Sandstones: GEOLOGIC NOTES

A unified hypothesis concerning generation and migration of hydrocarbons and development of secondary porosity in sandstones is presented. Carbonic acid is considered to be the primary reagent responsible for dissolution in the development of secondary porosity in sandstones. The degree to which reservoir quality is enhanced by development of secondary porosity may be proportional to the amount of hyrocarbons generated as well as to the amount of unstable constituents during diagenesis. In some cases, formation of secondary porosity may be responsible not only for development of reservoir but also for a significant element of entrapment by the partial but areally and stratigraphically selective dissolution of sandstone in the migration path of hydrocarbons. (JMT)

Importance of Secondary Porosity in Sandstones to Hydrocarbon Exploration: ABSTRACT

Importance of Secondary Porosity in Sandstones to Hydrocarbon Exploration: ABSTRACT