A Continuous Uninterrupted Piece of a Single Mineral
Lithic Sandstone
lithic sandstone, fine to medium-grained, moderately sorted and immature, and composed predominantly of quartz, olivine, serpentine, pyroxene and iron oxides The conglomerate and breccia were depositedas alluvial fanglomerates and wadi deposits, while the lateritic and lithic sediments were deposited in a fluvial and marine environment.
From: Sedimentary Basins and Petroleum Geology of the Middle East , 2003
Overburden response to longwall mining
Hua Guo , ... Deepak Adhikary , in Advances in Coal Mine Ground Control, 2017
5.2.1 Stratified rocks
Sedimentary rocks are formed by the deposition and subsequent cementation of materials on Earth's surface and within bodies of water. Examples of sedimentary rocks include chalk, limestone, shale, quartz-lithic sandstone, conglomerate, chert, carbonaceous mudstone, and coal seams. Forming more than nine-tenths of Earth's surface, sedimentary rocks are important sources of fossil fuels, such as coal, oil, and natural gas.
Stratification, or division into layers, is the most persistent and conspicuous characteristic of sedimentary rocks (Fig. 5.1). A single member, or bed, of a stratified rock is called a layer, whether thick or thin. Each layer represents an uninterrupted deposition of material. Bedding planes are formed due to longer or shorter pauses in the deposition process, or to a change in the source material. A stratum is a collection of layers of the same mineral substance that occurs together. The passage from one stratum to another is generally abrupt, and indicates a change in the circumstances of deposition, either in the depth of water, the character of the material brought to a given spot, or both. If conditions remain the same for a considerable period, significant thicknesses of similar material may be formed, such as massive sandstone layers.
Figure 5.1. Stratified rocks exposed on the slope face of an open-cut mine.
Detailed characteristics of stratified rocks, such as lithology, structural features and mechanical properties, can be obtained from exploration boreholes. Fig. 5.2A shows a core obtained from an exploration borehole at an underground coal mine in Australia. The lithology can be clearly identified from the core, e.g., fine sandstones, medium sandstones, siltstones, tuffs, mudstones, and coal plies. Such visual geological interpretation can be combined with geotechnical analysis of exploratory geophysical logs to develop mine site-specific three-dimensional (3D) geological models that can assist in mine planning and design (Guo et al., 2000; Zhou et al., 2001; Guo and Zhou, 2011).
Figure 5.2. (A) A sample of borehole cores with recognizable geological units, (B) geotechnical strata classification using geophysical logs; for comparison, log-interpreted strata classification with core photographs for part of the hole is also shown (right-hand plot), (C) a perspective view of a 3D geotechnical model derived from geophysical log data interpretation. BA, BB, BM, and DM, coal plies; FN, fine grained sandstones; MD, medium grained sandstones; ST, siltstones; TF, tuffs; XM, carbonaceous mudstones.
Fig. 5.2B shows an example of the interpretation of lithological and geotechnical units derived directly from the geophysical logs for an Australian coal mine. In this example, the rocks are classified into geotechnical units in terms of coal, strong (sandstones), moderately strong (sandstones and siltstones) and weak strata (siltstones and mudstones). Geotechnical units are derived from density, gamma ray, and uniaxial compressive strength logs (derived from the sonic log), using the automated log interpretation software LogTrans (Fullagar et al., 1999). These geophysically derived geotechnical units can be used as input to generate a 3D block model for mine design, as described in Guo et al. (2000) and shown in Fig. 5.2C.
Stratified rocks can be highly anisotropic in their mechanical strength and deformation characteristics. The shear strength along the bedding plane, is generally low, and varies depending on surface profile, lithology, and infilling materials.
The groundwater flow system within stratified rocks is also anisotropic, with high hydraulic conductivity in the plane of bedding compared with that normal to bedding. Coal seams, sandstone and siltstone units generally possess high water storage capacity and higher permeability, and so are often characterized as aquifers. Shale and mudstone units generally have lower permeability and are characterized as aquitards.
Stratification has a major influence on the overburden response to longwall mining. It directly affects mine stability, surface subsidence, aquifer interference, mine water inflow, and mine gas emission and flow dynamics.
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The Cretaceous System
D.A. BROWN , ... K.A.W. CROOK , in The Geological Evolution of Australia and New Zealand, 1968
EASTERN SEABOARD
The oldest marine faunas (Neocomian) occur in sandstones near Stanwell west of Rockhampton (Hill and Denmead, eds., 1960). Nearby, and again a little further north in the Styx Basin , lithic sandstones and shales with coal seams (Styx Coal Measures) occur. One marine interbed containing Albian microplankton is known in this sequence.
To the south in the Maryborough Basin the Cretaceous commences with 1350 m of andesitic volcanics (Grahams Creek) including lavas, tuffs, and their re-worked derivatives, together with some trachyte and rhyolite. The Grahams Creek Volcanics probably extend to sea northeastwards for at least 80 km, and occur in the base of the Wreck Island No. 1 Well (Derrington et al., 1960).
The volcanics lie unconformably on the Jurassic Tiaro Coal Measures and are overlain conformably by 1800 m of marine shales and cherts of Aptian age (Maryborough Formation). A reversion to terrestrial conditions took place in the Albian, and the resultant Burrum Coal Measures (1650 m) consist of lithic sandstone and siltstone with shales and bituminous coals.
Of these seaboard occurrences, those in the Styx Basin and near Stanwell are preserved by down faulting whereas those in the Maryborough Basin form part of the strongly folded belt of Mesozoic rocks which was formed during the Late Cretaceous Maryburian Orogeny.
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The Devonian System
D.A. BROWN , ... K.A.W. CROOK , in The Geological Evolution of Australia and New Zealand, 1968
CARNARVON BASIN
The late Givetian or early Frasnian saw further sedimentation in this basin. Rocks of this age outcrop only along the eastern edge of the basin, but they have been identified in exploratory wells sunk near the present coast line, so they probably have a continuous distribution across the basin. At the base of the sequence are thin arkosic and lithic sandstones which rest unconformably on Precambrian gneisses. Approximately 500 m of shallow-water sandstones, calcarenites, fossiliferous limestones, and dolomites (the Gneudna Formation) follow. They contain Disphyllum, Hexagonaria, Austrospirifer, Cyrtospirifer, Polygnathus, and Icriodus, which probably indicate a Frasnian age. The Munabia Sandstone and the Willaraddie Formation which complete the Devonian succession, are lithic and quartz sandstones and siltstones with some conglomerates. They total about 900 m, and are shallow-water, cross-stratified units devoid of fossils apart from a few lepidodendroid fragments.
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The Triassic System
D.A. BROWN , ... K.A.W. CROOK , in The Geological Evolution of Australia and New Zealand, 1968
TERRIGENOUS FACIES
The Middle and Upper Triassic sediments of the Sydney, Lorne, and Bowen Basins and Springsure–Corfield Shelf have much in common. The basal unit in each is dominantly quartz–rich sandstone with minor grey shale–the Gosford Formation and Hawkesbury Sandstone of the Sydney Basin, and the Clematis Sandstone of the Bowen Basin and Springsure–Corfield Shelf. The Wianamatta Group of the Sydney Basin and the Moolayember Formation of the Bowen Basin and Springsure–Corfield Shelf overlie the quartz sandstones. These units also are similar, both consisting of argillaceous rocks with some lithic sandstone, especially in the upper part of the sequence. Dark grey shale is typical of the Wianamatta Group, whereas olive-green to brown mudstones predominate in the Moolayember Formation. Coal seams are rare.
The distribution of the Clematis–Moolayember sequence is more limited than that of the Rewan, save for an extension southward from the Bowen Basin into New South Wales. The sequence is absent from southwestern Queensland, but a buried outlier occurs in the northeast of South Australia (Fig. 8.3). Thicknesses of less than 300 m are general except in the central part of the Bowen Basin where the sequence thickens rapidly to about 2400 m (Malone, 1964), the thickening occurring in the Moolayember Formation.
The Middle and Upper Triassic of the Sydney Basin is more limited in extent than the Lower, probably because of Cainozoic erosion. It reaches a maximum thickness of about 450 m near Camden, southwest of Sydney.
The Maryborough Basin and Esk Graben each contain more than 450 m of quartzose sandstone (the Myrtle Creek and Wivenhoe Sandstones respectively) similar to those of the Bowen and Sydney Basins. Some interbedded grey shales are present also.
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The Jurassic System
D.A. BROWN , ... K.A.W. CROOK , in The Geological Evolution of Australia and New Zealand, 1968
EASTERN AND SOUTHERN AUSTRALIA
The Early Jurassic saw the initiation of sedimentation in the Great Artesian Basin of Queensland on an erosion surface cut into Upper Triassic and older units. In the adjoining Clarence–Moreton and Mulgildie Basins deposition may have been continuous from the Late Triassic into the Jurassic. These basins seem to have been appendages of the Great Artesian Basin during the Jurassic. Cross-stratification measurements in the Clarence–Moreton Basin (Hill and Denmead, eds., 1960) suggest that streams flowed through it into the Great Artesian Basin, depositing quartz-rich sands (Marburg Sandstone). In the Great Artesian Basin coeval quartz-rich sands (Precipice Sandstone) were spread over the old land surface by an extensive river system. The sediments on the northeastern margin of the basin were derived from the west, northwest, and east (Jensen et al., 1964/61). Rivers appear to have gained access to the sea by way of the Mulgildie Basin (Fig. 9.2). The Precipice Sandstone is an important producer of oil at Moonie and natural gas near Roma.
Although sands continued to accumulate in the Clarence–Moreton Basin throughout the Early Jurassic (McElroy, 1963), finer material (Evergreen Shale) with some lithic sandstone succeeded the Precipice Sandstone in the Great Artesian and Mulgildie Basins. The Evergreen may be marine in part, for swarms of acritarch hystrichospheres occur at several localities ( Evans, 1962/115).
A return to fluviatile conditions probably occurred with the deposition of the quartz-rich Hutton Sandstone which is the most widespread of the Lower Jurassic units, extending from Queensland into New South Wales and South Australia (Fig. 9.2).
Minor extrusions of basalt took place at this time in the Clarence–Moreton Basin (Towallum Basalt). Probably contemporaneous volcanism also occurred in the southernmost part of the Great Artesian Basin (Garrawilla Volcanics) (Dulhunty, 1965). The latter reach 180 m in thickness. The maximum thickness of the Lower Jurassic sequence in each basin is about 600 m. This thickness is reached in the Great Artesian Basin only in the eastern part.
The Middle Jurassic saw a marked increase in the area of sedimentation in the Great Artesian Basin (Fig. 9.3). Throughout this basin, and the Mulgildie and Clarence–Moreton Basins, a sequence of lithic sandstone, shale, sideritic mudstone and sub-bituminous coal (Walloon and Mulgildie Coal Measures) accumulated, apparently under fluviatile to swamp conditions. The Walloon reaches a maximum thickness of 600 m in the Clarence–Moreton Basin where it extends into the Upper Jurassic at least. Elsewhere it is confined to the Middle Jurassic, and rarely exceeds 300 m, being much thinner towards the margins of the Great Artesian Basin.
By Late Jurassic times the area of sedimentation in the Great Artesian Basin had extended over the shelving borderlands of the basin, where thin sandstone-shale sequences were deposited. On the "Boulia Shelf", which borders the Precambrian inlier of western Queensland, the Longsight Sandstone accumulated. Equivalent strata west of Lake Eyre form the Algebuckina Sandstone. Deposition also occurred in the Lake Frome Embayment in South Australia, and on the margins of the Eulo Shelf in New South Wales and Queensland. Probably much of this marginal deposition took place near the close of the Jurassic.
The Upper Jurassic sequence in the Great Artesian Basin is best known in Queensland, and comprises three units, the basal Gubberamunda Sandstone, the Orallo Formation (formerly known as the "Fossil Wood Beds"), and the Mooga Sandstone Member of the Blythesdale Formation (Day, 1964). Previously these have been regarded as Cretaceous, but palynological studies (Evans, 1963) suggest that their age is Late Jurassic. These units are recognized throughout the basin, although near its margins all but the Mooga may be missing. Over much of the basin where all three units are present, they have an aggregate thickness of about 450 m. Their maximum thickness of about 1500 m occurs in the eastern part of the basin near Cabawin. They are entirely terrestrial, probably largely fluviatile. The sandstones in the Orallo Formation are lithic whereas those in the Gubberamunda Sandstone and Mooga Sandstone Member are quartzose. Shale and thin coal seams occur in the two upper units, particularly in the Orallo.
The Algebuckina Sandstone of South Australia, which is probably an equivalent of the Mooga, contains well-rounded quartz grains and dreikanter and locally exhibits very large-scale cross-stratification (Wopfner, pers. comm.). These features indicate aeolian deposition and its location in the interior of the continent suggests local desert conditions.
Part of the Walloon Coal Measures in the Clarence–Moreton Basin is known, on microfloral evidence, to be coeval with the lower part of the Blythesdale Group in the Great Artesian Basin, but the precise age limits of the Walloon as a whole are in doubt. It may be wholly Jurassic, or the upper part may be Lower Cretaceous, as young as Aptian or even Albian (Rade, 1964).
The possible equivalents of the Orallo and Mooga in the Clarence–Moreton Basin are the Kangaroo Creek Sandstone and the overlying Grafton Formation, which have a combined thickness of 400 m. The Kangaroo Creek Sandstone is quartz-rich. It lies, probably unconformably, on the Walloon Coal Measures. The movement that produced this unconformity may be expressed further north by the absence of Upper Jurassic strata from the Maryborough Basin. The Grafton Formation resembles the Orallo Formation, consisting of lithic sandstone, siltstone, claystone, and minor sub-bituminous coal seams.
The Jurassic System in the southern part of South Australia is confined to the small Polda Basin on the west side of Eyre Peninsula (Fig. 9.1), which contains 75 m of terrestrial lignitic and micaceous silts and clays with an Upper Jurassic microfloral assemblage (Harris, 1964).
Sedimentation commenced in the Laura Basin of north Queensland after the start of Jurassic time. The lower parts of the Dalrymple Sandstone, predominantly a quartz sandstone with some conglomerate and shale, which reaches about 450 m in thickness, are non-marine–probably fluviatile and lacustrine–and were derived largely from the eastern side of the basin (Lucas, 1962/149, Lucas, 1964/93). The presence of some microplankton high in the Lower Jurassic sequence suggests some marine influence in the depositional environment at this time. During the remainder of the Jurassic the Dalrymple Sandstone continued to accumulate under non-marine conditions. It is overlain, disconformably near the margins of the basin, by Lower Cretaceous strata.
Although widely separated geographically, the Maryborough and Carpentaria Basins have in common the absence of recognized Lower and Upper Jurassic strata. Subsurface studies in the Carpentaria Basin may yet reveal Upper Jurassic strata, however.
The Middle Jurassic of both basins is a coal-bearing sequence. That of the Carpentaria contains much sandstone and is similar to the sequence in the Morehead Basin of southwestern Papua (A.P.C., 1961). The Tiaro Coal Measures of the Maryborough Basin lie disconformably on the Triassic Myrtle Creek Sandstone. They comprise an unknown thickness of coals, carbonaceous shales and lithic sandstones, much disturbed by faulting, folding and igneous intrusions.
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INFRACAMBRIAN OF THE MIDDLE EAST
A.S. ALSHARHAN , A.E.M. NAIRN , in Sedimentary Basins and Petroleum Geology of the Middle East, 2003
Comparison with the Republic of Yemen
Continuing southwest from southern Oman, an Infracambrian sequence is found in southeastern Yemen in the Hadhramout, west of Mukalla. It is unmetamorphosed or virtually unmetamorphosed. Beydoun (1964, 1966) recognized and briefly described four relatively unfossiliferous formations collectively referred to as the Ghabar Group (Figs. 3.2 and 3.11). They crop out in two main areas: the Wadi Ghabar and Minhamir. Although lithological and thickness changes are seen between the two main areas of outcrop, they bear some similarities to the Huqf Succession in Oman, and correlation is based on lithological arguments (Gorin et al., 1982). Although no equivalent of the Ara evaporite sequence is seen, gypsum is reported in the uppermost, Harut Formation, providing some indication of arid conditions, and the four formations of Beydoun show the same alternation of clastic and carbonate sequences.
Fig. 3.11. Sedimentological interpretation of the Infracambrian-Early Cambrian (Ghabar Group) in the extreme eastern part of Yemen.
(modified from Beydoun, 1966) Copyright © 1966Acidic and basic rocks intruding the rocks of the Ghabar Group have been dated 590±50 Ma, supporting the correlation with the Huqf Group. The most reasonable parallels with the Huqf Group are to equate the Shabb with the Khufai, and the Harut with the Buah, suggesting that a more detailed examination should reveal the presence of stromatolites.
The base of the Minhamir Formation rests on a peneplaned, crystalline basement. The formation, which reaches a thickness of 143 m (469 ft), consists of tuffaceous, limey, conglomeratic sandstone, lithic tuffs and tuffaceous mudstone containing a variety of lava pebbles, mostly of intermediate character with rare metamorphic and chert pebbles probably of fluvial origin (Greenwood and Bleakley, 1967; Beydoun and Greenwood, 1968).
The Shabb Formation (Fig. 3.11), which conformably overlies the Minhamir, is made up of mainly thin-bedded, occasionally sandy limestone, with some chert bands and gypsum occurring parallel to bedding. Sand forms are an increasingly important component in the upper part of the formation. The thickness diminishes from 43 m (141 ft) in Wadi Ghabar to only 13 m (43 ft) in Wadi Minhamir. Deposition appears to have occurred in a restricted environment, with conditions verging on an evaporitic setting (Beydoun, 1966; Beydoun and Greenwood, 1968).
The Khabla Formation shows a gradual transition up from the Shabb Formation and consists of 210 m (688 ft) of dolomites, calcareous sandstone, mudstone, calcareous and dolomitic siltstone with some interbedded dolomite, and gypsum suggesting deposition in a shallow-marine, slightly restricted environment.
The Harut Formation also shows a transitional passage up from the underlying Khabla beds. In Wadi Ghabar, it consists of 31 m (102 ft) of well-bedded, sandy dolomites; massive, calcareous quartzites; some platey, shale beds; impure dolomite; and fairly pure limestone. It is somewhat thicker (48 m, or 157 ft) in Wadi Minhamir, where it consists of gypsiferous, sandy and silty limestone. The sand grains within the carbonates are rounded, suggesting the existence of a shallow-marine restricted in arid environment (Beydoun, 1966).
In the southern part of the former South Yemen, two metamorphic groups, the Garish and Thaniya groups, appeared correlatable with the Ghabar Group. The first, the Garish Group, found in southwestern South Yemen, consists of a sedimentary sequence of rocks that have undergone metamorphism to an albite-epidote-amphibolite grade, with the grade of metamorphism increasing toward the west (Beydoun, 1964, 1966). The second, the Thaniya Group, crops out in western South Yemen, with the protoliths, sandstone, siltstone, shale and limestone metamorphosed to an albite-epidote-hornblende facies (Beydoun, 1964, 1966).
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The Permian System
D.A. BROWN , ... K.A.W. CROOK , in The Geological Evolution of Australia and New Zealand, 1968
EXTRUSIVES
Four volcanic units can be recognized. (1) The North Queensland Province in which felsites, rhyolites, and ignimbrites cover several thousands of square kilometres and lie unconformably on the middle Palaeozoics of the Hodgkinson and Bundock Basins, across the Tasman Line, and on to the Precambrian of the Georgetown Massif. In most places these rocks are undated, but at a few localities they have been found conformably overlying sandstones with a Glossopteris flora (Hill and Denmead, eds., 1960) . (2) In the Newcastle Geosyncline there is a predominance of volcanics in the Lower Permian. Local volcanic sequences may be almost entirely basaltic as in the Hunter Valley or on the northwestern edge of the Bowen Basin, basaltic and rhyolitic in northern New South Wales, andesites as in the Dawson Valley, or interstratified andesites, rhyolites, and trachytes as along most of the northeastern edge of the Bowen Basin. Many of the lithic sandstones in this structure contain material of volcanic origin which is often stated to be tuffaceous but which may equally be due to the degradation of a pre-existing volcanic terrain. Extensive Late Permian volcanism was present only on the south coast of New South Wales. (3) The Yarrol Trough has a long volcanic history, and in places there are thousands of metres of Upper Permian andesites and basalts. The equivalents of the Lower Bowen Volcanics are developed only locally. (4) Volcanic rocks were extruded at various times during the Permian on and in basins along the eastern flanks of the South Coastal-New England High. Rhyolites and acid porphyries together with acid tuffs form the bulk of these rocks, but andesite and andesitic tuff is also particularly abundant in some areas, for instance the Mary borough Basin.
INTRUSIVES
The Permian–Early Triassic was a period of intense plutonic activity in northeastern New South Wales and eastern and northeastern Queensland. No plutons of proven Permian age are found outside this region. Most of the rocks are acid, mainly adamellites, but there are also other acid and intermediate types. Most have been intruded into rocks of the Tasman Orthogeosyncline, though in north Queensland they also intrude the Precambrian Georgetown Massif. The bulk of them were intruded into the New England High or the Eungella–Gogango High, but it is probable that small bodies were also intruded into the Bowen Basin sediments themselves at this time. All are structurally discordant, intruded at a high level in the crust, and produce only narrow contact aureoles. In New England and southern Queensland the New England Batholith has been shown to be composite. The sequence of intrusions does not follow the acid to basic pattern, and on the contrary some are exactly the opposite. This is considered to be the result of progressive hybridization (Chappell, pers. comm.).
Of particular interest are several stocks of granite in north Queensland which were intruded to very high levels in the crust, and in places broke the surface to form rhyolites. The base of these extrusives was then intruded by the granite. Elsewhere the granites are "hooded" by flow-banded porphyry (Hill and Denmead, eds., 1960).
Little is known of the time relations of these rocks since most of them are dated in the most general way only. An upper limit is set in some places to the north of Brisbane where granites are faulted against Lower Triassic rocks and are overlain by Jurassic sandstones. Radiogenic dates from New England and from the eastern side of the Bowen Basin suggest that the period of intrusion ranged from Early to Late Permian. Some plutons, for example, in the Hillgrove district, may be Carboniferous in age (Cooper et al., 1963).
Serpentinites are found in contact with Permian rocks in the Great Serpentine Belt of New South Wales and its continuation into Queensland (the Yarrol thrust zone), as well as in the area near Taree. At one locality near Taree the serpentinites occupy a fault zone between Upper Carboniferous and Permian sediments. It is thought possible that these relationships are the result of re-intrusion of earlier serpentinite, but this is not proved.
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Reconstruction of Sag-Wide Reservoir Characteristics
Xianzheng Zhao , ... Xiugang Pu , in Re-exploration Programs for Petroleum-Rich Sags in Rift Basins, 2018
Origin mechanism and model of good reservoirs at steep slopes
The steep slope category includes the Jiuzhou-Gu'an structure zone of the Langgu Sag, and the Chaheji-Gaojiapu-Maozhou area of the Baxian Sag. Comparing it with gentle slopes, the main controlling factors for steep slopes are the composition of sediments and formation overpressure at the early stage, besides sedimentary facies, diagenesis, and early charging.
Subaqueous fan facies are developed in the Paleogene formation in the Jiuzhou-Gu'an structure zone. Gravity flow deposits are rapidly heaped up. Conglomerate is the main rock type, occurring as tangential contacts and matrix support (Fig. 5.18A–D ). Poorly sorted and poorly-moderately sorted, sub-grounded to sub-angular, and a high matrix are the other characteristics (Fig. 5.18E). A complex composition of gravels is found, including carbonate and clastic gravels. The content of carbonate gravel is over 90%. Constrained by these particular sedimentary facies and components, the reservoir space is composed of few intergravel pores (Fig. 5.18A), inherited intergravel dissolution pores (Fig. 5.18D and F) and fractures (Fig. 5.18G). Generally, the reservoir properties are poor.
Fig. 5.18. Reservoir characteristic of Jiuzhou-Gu'an structure zone in the Langgu Sag.
The textures of different microfacies in the underwater fan are different. The main channel of inner fan has coarse-grained, poorly sorted deposits, mainly supported by a matrix. The braided channel of the mid-fan is largely composed of fine-grained sediments, with a low content of matrix and supported by particles. The interchannel of the mid-fan mainly consists of poorly sorted and fine-grained deposits with a high content of matrix. The difference in physical properties between sedimentary facies is evident (Table 5.7).
Table 5.7. Physical properties of different microfacies in Jiuzhou-Gu'an structure zone
| Sedimentary Microfacies | Number of Data/Number of Effective Data | Porosity Difference (%) | Number of Data/Number of Effective Data | Permeability Difference (mD) | ||||
|---|---|---|---|---|---|---|---|---|
| Min | Max | Avg | Min | Max | Avg | |||
| Braided channel | 271/141 | − 8.28 | 24.16 | 1.34 | 68/19 | − 2.23 | 45.05 | 0.69 |
| Main channel | 23/7 | − 12.95 | 18.37 | − 0.5 | – | – | – | – |
| Interchannel | 22/3 | − 8.68 | 0.59 | − 3.44 | 5/0 | − 2.24 | − 1.25 | − 1.88 |
Meanwhile, the reservoir properties of the conglomerate are affected by the composition of gravels. Dolomitic conglomerate has a porosity of 5%–15% and a permeability of greater than 1 mD (Fig. 5.19A ). Limestone conglomerate has a porosity of less than 10% and a permeability of less than 1 mD (Fig. 5.19 B). Apparently, dolomitic conglomerate has better properties than limestone conglomerate. Some research reveals as the cause that dolomite has well-developed intercrystalline pore and is brittle. First, dolomite usually has crystals, while calcite is more likely to be microlitic. Enormous intercrystalline pores are created during the dolomitization and recrystallization of dolomite crystals. Within a certain crystal size, intercrystalline pore is growing when the crystals are growing bigger. It explains that dolomite usually has more pores than calcite and more easily forms dissolution pore. Second, dolomite has a higher anticompression capability and greater brittleness than calcite, which means that at a given pressure, dolomite can preserve pores better or dolomite can generate fractures easier. Hence, dolomitic conglomerate has more factures and pores than limestone conglomerate, which indicates the better reservoir properties of dolomitic conglomerate. Fan delta deposits are the dominant facies found in the Chaheji-Gaojiapu-Maozhou area. The reservoirs are mainly composed of fine-grained sandstone and siltstone. The key rock types are feldspar sandstone and feldspar lithic sandstone, which are well sorted and moderately rounded. Different properties still exist in various microfacies of fan delta ( Table 5.8). Good reservoirs prefer to be formed at distributary channels of the delta front and mouth bar. The difference in average porosity is more than 2.4%. The difference in average porosity is more than 10 mD. Sheet sand and distributary bay microfacies are not favorable for the deposition of good reservoirs. The difference in average porosity is less than zero. Its efficiency is lower than channel and mouth bar.
Fig. 5.19. Physical properties of different types of conglomerate in the Jiuzhou-Gu'an structure zone. (A) Dolomitic conglomerate. (B) Limestone conglomerate.
Table 5.8. Reservoir properties of different microfacies of Paleogene mid-deep reservoirs in the Chaheji-Gaojiapu-Maozhou area
| Sedimentary Microfacies | Number of Data/Number of Effective Data | Porosity Difference (%) | Number of Data/Number of Effective Data | Permeability Difference (mD) | ||||
|---|---|---|---|---|---|---|---|---|
| Min | Max | Avg | Min | Max | Avg | |||
| Subaqueous distributary channel | 729/535 | − 6.75 | 20.4 | 2.39 | 365/218 | − 3.02 | 144.65 | 10.7 |
| Mouth bar | 68/53 | − 9.44 | 15.11 | 2.49 | 40/22 | − 0.96 | 268.1 | 12.85 |
| Sheet sand | 157/55 | − 7.79 | 20.6 | − 0.47 | 60/19 | − 3.23 | 47.65 | 4.11 |
| Subaqueous distributary bay | 133/10 | − 9.66 | 14.9 | − 3.30 | 33/3 | − 1.0 | 5.36 | − 0.37 |
At steep slopes, formation overpressure is formed at an early time, with good perseverance and is long lasting. The overpressure that occurred early could protect primary pores from compaction, which is beneficial for good reservoirs. As for the Chaheji-Gaojiapu-Maozhou area, around 43 Ma, formation overpressure was the largest (around 24.8 Ma) at Well Xinglong-1 (highest pressure factor = 1.667). Around 24.8 Ma, due to a slow deposition rate, pressure factor gets lower. However, medium to weak overpressure dominates since single the sandstone thickness is great and holds a high ratio, which helps preserve the formation pressure (Fig. 5.20). Based on the analysis of measured porosity of distributary channel reservoirs, normal pressure and weak overpressure reservoir (pressure factor is 0.9–1.2) have an average porosity difference of 2.1%. Medium overpressure reservoir (pressure factor is 1.2–1.5) has an average porosity difference of 2.77%. High overpressure (pressure factor is over 1.5) reservoir has an average porosity difference of 4.63%.
Fig. 5.20. Paleo-formation pressure evolution of Paleogene mid-deep formation in the Chaheji-Gaojiapu-Maozhou area (Well Xinglong-1). (A) Sha 4. (B) Sha 3. (C) Sha 2.
It could be seen that overpressure has a positive correlation with reservoir properties. Abnormally high pressure at an early time is helpful for the perseverance of primary pores while the existence of primary pores is useful for reservoir transformation by diagenetic fluids at a late time, creating intergranular dissolution pores or secondary pores.
By comparing it with the slope area, the control of diagenesis on steep slopes is demonstrated by secondary pores created by dissolution. Our research reveals that at the time of 39.6–28.3 Ma, the environment of mid-deep reservoirs in Chaheji-Gaojiapu-Maozhou area was gradually transformed to acid. With acidic fluids, dissolution would occur in unstable minerals such as feldspar, debris and carbonate cements, generating enormous secondary pores. The secondary porosity created by the dissolution in feldspar and rock fragments is 0.65%–16.07% (mean: 3.58%). The secondary porosity resulting from the dissolution of carbonate cements is 0.13%–1.28% (mean: 0.49%). With the increasing depth, the dissolution in reservoir becomes more intensive (Fig. 5.21).
Fig. 5.21. Secondary porosity variation in Chaheji-Gaojiapu-Maozhou area.
The impact of early charge on steep slopes is similar to the impact on gentle slopes. It shows that early charging can slow down the compaction and depress cementation, which is beneficial for preserving primary pores.
According to the statistics of well testing and reservoir differences, the porosity tendency of oil layer, oil-water layer, oil-bearing water, water layer, and dry layer appears to be similar to the one in the Wen'an slope: oil layer > oil-water layer > oil-bearing water layer > water layer > dry layer (Fig. 5.22).
Fig. 5.22. Porosity difference histogram of various oil columns in Paleogene mid-deep reservoir, Chaheji-Gaojiapu-Maozhou area.
Incorporated with the good reservoir origin mechanism analysis for the steep slope area, diagenetic evolution, and porosity evolution, a model for the formation mechanism of mid-deep good reservoirs in steep slopes has been established, by taking Chaheji-Gaojiapu-Maozhou area as an example (Fig. 5.23).
Fig. 5.23. Genetic model of Paleogene reservoirs in the Chaheji-Gaojiapu-Maozhou area.
As shown in Fig. 5.23, the mid-deep reservoir has experienced a diagenetic evolution, i.e., alkalinity-acidity-alkalinity-weak acidity, in the Chaheji-Gaojiapu-Maozhou area. Around 50.4–39.6 Ma, due to a dry and hot climate, and a closed water environment, the gypsum-salt layer was well developed. With an alkaline depositional aquifer, intensive carbonate cementation occurred around the contact between mudstone and sandstone, consuming lots of pore space. While in the sandstone, subtle carbonate cementation preserved large amounts of primary intergranular pores. Formation overpressure started at 43 Ma, restraining compaction to protect primary pores in the inner parts of sandstone bodies. Around 39.6–28.3 Ma, enormous organic acid was generated and then migrated to sands, dissolving some unstable minerals, such as feldspar and rock fragments. Secondary pores had been created to improve reservoir properties. During this period, formation overpressure further increased, and hydrocarbon charging started. Around 28.3–9.8 Ma, heat-induced decarboxylation occurred in the anion of organic acid and gypsum-salt layer started to expel a large amount of alkaline fluid. Besides, an alkali metal ion, generated during the transformation of smectite to illite, entered into pore fluids in the mudstone. The formation water was changed to alkaline and carbonate cementation occurred. For those sand bodies with hydrocarbon charging during the early time, cementation is very limited during the late time. The formation was still moderately highly over pressed. Around 9.8 Ma, the temperature of formation was more than 140°C and organic acid totally decomposed. At the same time, the content of CO2 increased and the dehydration in the gypsum-salt layer stopped. Since the alkali metal ion had been consumed significantly during carbonate cementation, the surplus CO3 2– and HCO3 – in the formation water were transformed to weak acidic. Very few feldspar and carbonate cements dissolved, which did not have an obvious improvement on reservoir properties.
In the Jiuzhou-Gu'an structure zone, the genetic model of conglomerate reservoir is relatively simple, which is mainly influenced by original sediments and tectonic movements in the later stage, very little by diagenesis. Grain-supported dolomitic conglomerate is the dominant rock type of good reservoirs, which develops inherited matrix pores and dissolution pores.
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https://www.sciencedirect.com/science/article/pii/B9780128161531000055
Source: https://www.sciencedirect.com/topics/engineering/lithic-sandstone