Unstable Grains Research Articles

DIAGENESIS AND RESERVOIR QUALITY, TOOLACHEE FORMATION, COOPER BASIN

A preliminary petrological study of the Toolachee Formation in three Cooper Basin wells indicates that the intensity of diagenetic changes in the sandstones is the main factor influencing reservoir quality.Reduction of primary porosity occurs as a result of silica cementation as quartz overgrowths, dissolution of quartz grains forming microstylolitic grain contacts, authigenic clay mineral formation accompanying breakdown of chemically unstable grains, and cementation by siderite. Secondary porosity is erratically developed as a result of dissolution of some unstable grains particularly in rocks showing an advanced state of diagenetic alteration. High permeabilities are obtained only from those sandstones where significant primary porosity is still retained.Removal of chemically unstable grains during diagenesis has resulted in the formation of mineralogically mature sandstones, which consist almost entirely of quartz and clay minerals. The only other detrital components present are chemically resistant minerals such as muscovite or rock fragments composed of these resistant minerals.A relatively simple relationship of increasing intensity of diagenetic processes with increasing depth of burial may exist for the Toolachee Formation in the Cooper Basin.

Geologic and Geophysical Study of Cerro Prieto Geothermal Field, Mexico: ABSTRACT

The Cerro Prieto geothermal field is near the southwestern margin of the Colorado River delta, Baja California. The subsurface stratigraphy at Cerro Prieto is characterized by complex vertical and lateral variations in lithofacies, which is typical of deltaic deposits. The geothermal production zone is not a uniform reservoir layer overlain by a laterally continuous top seal of low-permeability strata. The top of the geothermal-related hydrothermal alteration zone has a dome-like configuration which cuts across the sedimentary strata. Shales in the altered zone exhibit high densities and high resistivities on the well logs relative to those outside the zone. The geothermal producing intervals generally straddle or underlie the top of the altered shale zone. Sandstones in the hydrothermal alteration zone commonly have fair to good porosities (15 to 35% or higher), which have resulted from the removal of unstable grains and carbonate cement by solution. Open fractures are unusual in the altered zone, based on core description. While fractures may be an important contributor to local reservoir permeability, secondary matrix porosity and permeability are considered to be more important volumetrically in the Cerro Prieto reservoirs. Detection of geothermal anomalies in the Cerro Prieto region may be difficult from resistivity, magnetic, or gravity data. However, the occurrence of a reflection-poor zone coincident with the hydrothermal alteration zone suggests that the seismic reflection method may be a good approach to detecting these anomalies. Other types of geophysical data are necessary to eliminate alternate causes of reflection-poor zones on seismic profiles. End_of_Article - Last_Page 951------------

Petrology and Diagenesis of Early Miocene Deep-Sea Fan Deposits near Point Arena, California: ABSTRACT

Early Miocene deep-sea fan deposits of the Skooner Gulch and Gallaway Formations near Point Arena, California, were examined to determine which factors influence or control diagenesis in fine-grained turbidite deposits. Well-preserved sedimentary structures (including complete Bouma Ta-e beds) are nearly continuously exposed in a vertical sequence along the coast. This sequence is interpreted to represent outer fan depositional lobes, lobe fringe, and basin plain deposits. Four representative stratigraphic sections were sampled and analyzed using the scanning electron microscope, energy dispersive X-ray, X-ray diffraction, nuclear magnetic resonance, electron microprobe, and thin section analysis. Rock types examined include medium to very fine-grained arkoses, lithic arkoses, mudstones, siltstones, and hemipelagites. Diagenesis of a turbidite sequence is complex and does not involve uniform changes in detrital and authigenic minerals. Factors affecting diagenesis include variations in the original depositional environment, sediment composition, texture, stratification, frequency of interbedded shale beds, bioturbation, degree of compaction, and original porosity and permeability. Diagenetic changes recognized were: (1) alteration and resorption of unstable detrital grains, such as feldspar, rock fragments and detrital clay; (2) formation of authigenic quartz, potassium feldspar, and clay cements which reduced porosity and permeability; (3) formation of iron oxide and pyrite; and (4) incorporation of detrital and authigenic minerals in late-stage carbonate cement. All of the observed diagenetic cha ges can occur at relatively low temperatures and shallow to moderate burial depths. End_of_Article - Last_Page 1007------------

Sandstone in Lower Cretaceous Helvetiafjellet Formation, Svalbard: Bearing on Reservoir Potential of Barents Shelf

The Helvetiafjellet Formation consists of 50 to 100 m of largely fluvial sandstones and shales deposited throughout Svalbard during a major regression. Sandstones consist chiefly of monocrystalline and polycrystalline quartz, feldspar, chert, and volcanic (basaltic) rock fragments in (1) quartz and quartzose arenites, (2) quartzose lithic arenites, and (3) volcanic arenites. Quartz and quartzose arenites dominate in western Spitsbergen and were derived from a mixed sedimentary-metamorphic terrane in the west. Volcanic arenites are present in Kong Karls Land in the east, and are intercalated with basaltic lava flows which are considered to be the major source for the sandstones. Quartzose lithic arenites in southeast Spitsbergen are a mixture of quartz sands derived from the west and reworked volcanic sands transported from the east. Cementing minerals are variably developed in these sandstones. Quartz overgrowths are dominant in quartz and quartzose arenites. Kaolinite is present in many feldspar-bearing quartz and quartzose arenites, whereas illite is rare. Chlorite is common, together with calcite, in quartzose lithic arenites. Abundant smectite and calcite with minor zeolites are present in the volcanic arenites, which suggests a strong relation between mineralogy of the detrital grains and that of associated cements. Unstable detrital grains released ions to pore waters, which subsequently precipitated clay and calcite cements. Induration increases and maximum porosity decreases from east to west across Svalbard, owing both to higher temperature, caused by progressively greater burial depth and steeper geothe mal gradients, and to tectonic stress in westernmost areas. Early Cretaceous volcanic activity may have provided abundant chemically unstable detritus to basins in the southern part of the Barents Shelf. As a major consequence, clay cements could form during diagenesis, adversely affecting sandstone reservoir parameters. However, the development of secondary dissolution porosity could improve reservoir characteristics of those sandstones which contain chemically unstable minerals such as feldspar grains and calcite cement.

Diagenesis and Porosity Distribution in Deltaic Sandstone, Strawn Series (Pennsylvanian), North-Central Texas: ABSTRACT

ABSTRACT Extensive diagenetic changes have occurred in deltaic sandstones of the "Gray" interval, Strawn Series (Pennsylvanian) from Taylor County, Texas. The present burial depth of the "Gray" interval is 4500 feet (1370 m), and maximum burial was about 7500 feet (2300 m). Sandstone beds were deposited in delta slope, bar crest, and distributary channel environments in a high-constructive elongate delta. Petrographically, the rocks are fine to very fine, submature to mature, shale-bearing sublitharenite. Except for clay clasts, the framework grains, more than 95% quartz, are very mature mineralogically. Good primary porosity occurred in sediments deposited in high energy environments such as bar crest and channel. These sediments had large mean grain size (x = 2.8 or .15 mm), contained an average of only 1.49% detrital clay matrix, and were located farther away vertically from shale beds. It is estimated that depositional porosity of about 35% was reduced to 22% by compaction. Cementation began with authigenic chlorite rims, followed by quartz overgrowths. Calcite cement filled the remaining pore space and replaced unstable grains such as feldspar. Later, dissolution of calcite produced secondary porosity. Further compaction was prevented by quartz cement, so the secondary porosity was preserved. Kaolinite, barite, and ferroan dolomite precipitated as late cements that reduced secondary porosity. Samples from high energy environments contain the most cement, but they also retain the highest porosity. Original textures and depositional environments in a formation may be used to predict where porosity will survive diagenesis.

Sandstone Composition and Depositional Environment

The detrital mineralogy of sandstones is affected significantly by depositional environment. The imprint of the environment is recorded as the relative abundance and size of individual detrital species. Differences between environments within any one basin are accompanied by concomitant changes in detrital-sediment composition. Petrographic analyses of Holocene and ancient sedimentary sequences reveal that the environmentally produced compositional changes are process dependent, and are interrelated closely with textural variations. Fluctuations in rock-fragment content among environments indicate that the relative survivability of unstable detrital grains can influence the overall composition of sandstones. A total of 300 samples of varied grain sizes was selected from contrasting environments in three Holocene sedimentary sequences: (1) the Galveston barrier complex, Texas; (2) the lower alluvial valley of the Mississippi River; and (3) the Mississippi delta. In each sedimentary sequence, bivariate and multivariate analyses of petrographic data demonstrate the environmental sensitivity of both composition and texture. A comparison of data from these three Holocene sequences demonstrates that the most effective environmental segregation based on composition and texture alone is in depositional systems characterized by intermediate or high energy--in systems with a significant proportion of sand-size detritus. Four ancient sedimentary sequences, of different age and geographic location, w re selected to complement the Holocene sequences: (1) Wilcox (Eocene) fluvial rocks, Texas; (2) Wilcox (Eocene) deltaic rocks, Texas; (3) Lower Jurassic (Toarcian) marine rocks, England; and (4) Lower Cretaceous Muddy Sandstone marine, transitional, and nonmarine rocks, northeast Powder River basin, Montana and Wyoming. Petrographic data from each sequence demonstrate that the composition of recent detrital sediments and ancient sandstones is environmentally sensitive. Composition is also sensitive to textural changes. In sediments with a wide range of grain sizes, the relation between the abundance and size of detrital monocrystalline quartz is curvilinear. Compositional and textural data derived exclusively from thin-section analyses can be used jointly to segregate depositional environments within a single sedimentary basin, providing no source changes occur, and providing that certain basic sampling restrictions are adhered to. Thus, small samples such as sidewall cores and cuttings may be used in the delineation of depositional environments. Such small samples are available in abundance in the p troleum industry and, if analyzed petrographically, they can render invaluable assistance in the search for stratigraphic traps.

Petrology and Sedimentation of Early Paleozoic Island-Arc Deposits, Newfoundland: ABSTRACT

The Cambro-Ordovician New Bay Formation (2,000 m) of the Newfoundland Appalachians is a southward-thickening prism of conglomeratic, volcanogenic turbidites interbedded with sandstone and black argillite. Abundant sedimentary structures include tool and scour marks, cross-lamination, normal and multiple graded bedding, slump folds, and channels. The top of the formation is mostly agglomerate interbedded with red-and-green laminated silty argillite. Trails and burrows are the only fossils. The sandstones are lithic-feldspathic arenites and basaltic wackes composed of fresh, angular unstable mineral grains and little quartz. The conglomerates have volcanic and sedimentary clasts enclosed in an ashy matrix altered to chlorite. Volcanic clasts are basalt, andesite, and dacite porphyry. Sedimentary clasts include green cherty siltite, fine-grained feldspathic arenite, abundant rip-up clasts of argillite, and rare jasper. Analysis of Bouma sequences, amalgamated contacts, and trace fossils indicates abrupt regression late in New Bay deposition. Sole marks, slump folds, and channel axes indicate marginal sediment supply from turbidity currents and slumping down an east-facing submarine slope. On the northwest the formation thins and apparently passes into marine and subaerial pyroclastic strata of the Wild Bight Group. On the east it is in sedimentary and tectonic contact with the Dunnage melange which has been interpreted as an ocean trench deposit. The petrology and sedimentology of the New Bay Formation compares closely with certain Miocene volcanic arc deposits described from Malekula Island, New Hebrides. Therefore the New Bay is interpreted as a volcanogenic apron deposited in the arc-trench gap of a Cambro-Ordovician island arc. End_of_Article - Last_Page 779------------

Clay Mineralogy of Weathered Products and of River Sediments, Puerto Rico

ABSTRACT The island of Puerto Rico is composed primarily of an east-west mountain range of andesitic to basaltic volcanics with lesser amounts of calcareous sediments along the north and south shores. The weathering of the volcanics in humid areas produces a soil composed of halloysite, gibbsite, and quartz, whereas weathering in more arid areas produces a soil rich in montmorillonite. The northward flowing Rio Manati drains a volcanic area largely covered by lateritic soils containing halloysite, gibbsite, and quartz; the mineralogy of the stream load, however, shows montmorillonite in addition to halloysite and quartz. The load of the south-flowing Inabon-Caomo river group (on the lee side of the island), derived from a petrologic source area similar to that of the Rio Manati, has a high montmorillonite content with lesser amounts of mixed-layer montmorillonite-vermiculite, vermiculite, and halloysite. The Rio Farajardo draining a volcanic area with high rainfall contains halloysite and vermiculite primarily. The Rio Guayanes, draining a generally wet, granodiorite area, contains kaolinite primarily with lesser amounts of vermiculite and montmorillonite. The Rio Guanajibo, draining an arid to wet, volcanic area, contains mixed-layer montmorillonite-vermiculite primarily, with lesser amounts of halloysite, chlorite, and serpentine. The rivers of Puerto Rico reflect the climate conditions in their watershed insofar as the characteristic top-soil components in their loads is concerned. The mineralogy of the stream load from lateritic source areas, however, consists of, in addition to halloysite, clay minerals such as montmorillonite which originate in lowermost soil horizons; these clay mineral do not represent phases at equilibrium with the climate. Because of the small volume of clay relative to the larger volume of mineralogically unstable volcanic sand in the rivers, however, the bulk sediment being deposited off-shore provides only slight indication of climatic conditions on-shore. This observation indicates that ancient source area climatic conditions inferred from clay fractions of sedimentary rocks are always tenuous, particularly in view of the fact that some (if not all) of the clay fraction of a rock may he derived from altered unstable sand grains.