Hairuo Qing

Multistage dolomitization in Rainbow buildups, Middle Devonian Keg River Formation, Alberta, Canada

ABSTRACT More than 80 partially to completely dolomitized buildups of Middle Devonian Keg River Formation occur in the Rainbow sub-basin of northwestern Alberta. In six buildups (A, B, E, F, G, and Tehze), four petrographic types of dolomites are identified: gray, fine crystalline dolomite; floating dolomite rhombs/patches; matrix dolomite; and saddle dolomite. The petrographic and geochemical data indicate that these dolomites probably formed in three principle stages of dolomitization during progressive burial. Gray, fine crystalline dolomite (less than 1% of total dolomite by volume) is interpreted as penecontemporaneously dolomitized lime muds by either normal marine waters or evaporitic brines during early exposure. The following evidence suggests that this dolomite probably formed early: 1) it usually occurs in fractures and breccias presumably related to the early exposure of the buildups; 2) it is absent in rugs and molds that formed in the subsurface environment; and 3) locally, clasts of gray, fine crystalline dolomite are embedded in marine limestones. Rhomb/patch dolomite is the major type of dolomite in dolomitic limestones and is widespread throughout the limestone buildups. Matrix dolomite is fabric destructive and is most abundant in the dolostone buildups. Both these dolomites probably formed during shallow to intermediate burial because they 1) postdate stylolites, 2) are widespread throughout the buildups and crosscut various facies, and 3) have light oxygen isotope compositions and low Sr but high Fe and Mn concentrations. Dolomitizing fluids for dolomite rhombs/patches and matrix dolomites were possibly derived from 1) mechanical compaction, 2) dehydration of the adjacent Muskeg gypsum at relatively shallow burial depths, and 3) intermediate-depth basinal fluids conducted updip along porous and permeable conduits. During D vonian and carly Carboniferous times, compaction fluids were probably responsible for the early dolomite rhombs and patches. However, the main phase of matrix dolomitization took place later, probably during Late Mississippian to Jurassic time when the east side of the basin was intermittently tilted and uplifted, causing updip flow of intermediate-depth basinal brines. Saddle dolomite occurs as late-stage vug and fracture fillings. It is most depleted in 18O but most enriched in Mn and Fe, suggesting precipitation during deeper burial from warm basinal fluids. These fluids were expelled during Upper Cretaceous sedimentary and tectonic loading of the western side of the basin and then moved up the east side of the basin along existing platform, fault, and fracture conduits. The source of the Mg is uncertain. Some or most of the Mg may have resulted from chemical compaction of the earlier matrix dolomite.

Low‐ and high‐frequency sea‐level changes control peritidal carbonate cycles, facies and dolomitization in the Rock of Gibraltar (Early Jurassic, Iberian Peninsula)

The Rock of Gibraltar comprises two tectonically separated limbs of an isolated klippe of Liassic Gibraltar Limestone Formation. Both limbs have similar, c. 400 m thick sequences of inner carbonate platform facies arranged in high‐frequency, metre‐scale, shallowing‐upward, peritidal cycles with emergent, caliche caps. Four cycle types are recognized on the basis of vertically repeated successions of different sedimentary structures, lithologies, facies and biota. When compared with other Liassic cycles from fault‐bound platforms of the western Mediterranean region all are found to be of similar scale, facies and cycle type. Likely common origins are through Milankovitch band allocyclicity, or autocyclic tidal flat progradation superimposed on regional subsidence. Within the Gibraltar Limestone high‐frequency cycles are superimposed on a low‐frequency (third order?) cyclicity that is revealed, through the use of Fischer plots, to control the occurrrence of facies, biota, high‐frequency cycle types and dolomitization. Falling sea‐level and lowstand phases, with reduced accommodation space, are typified by restricted, inner platform facies and cycles and by early reflux dolomitization. Transgressive and highstand phases, with more accommodation space, are characterized by the absence of early dolomites, the incoming of inner platform microfossils (i.e. foraminifera and calcareous algae) and by less restricted marine facies (i.e. oncoids, shelly rudstones, packstones and grainstones). Fischer plots have demonstrable value in the correlation and analysis of tectonically separated and geographically isolated cyclic sequences that lack prominent marker beds or stratigraphically useful biotas.

Well testing analysis for horizontal well with consideration of threshold pressure gradient in tight gas reservoirs

A fundamental solution for homogeneous reservoir in infinite space is derived by using the point source function with the consideration of the threshold pressure gradient. The fundamental solution of the continuous point source function is then derived based on the Green function. Various boundary conditions of the reservoirs are considered for this case and the corresponding solutions are obtained through the mirror image reflection and the principle of superimposition. The line source solution is obtained by integration. Subsequently, the horizontal-well bottom hole pressure response function for a non-linear gas flow in the homogeneous gas reservoir is obtained, and the response curve of the dimensionless bottom hole pressure and the derivative for a horizontal well in the homogeneous gas reservoir are obtained. In the end, the sensitivities of the relevant parameters are analyzed. The well test model presented in this paper can be used as the basis of the horizontal well test analysis for tight gas reservoirs.

Primary dolostone related to the Cretaceous lacustrine hydrothermal sedimentation in Qingxi sag, Jiuquan Basin on the northern Tibetan Plateau

Based on comprehensive studies in petrography, petrofabric analysis and geochemistry, this paper describes a unique and rare laminated micritic ferruginous primary dolostone crystallized and precipitated from the alkaline hot brine under the conditions of the Mesozoic faulted lake basin. The main rock-forming mineral of this dolostone is ferruginous dolomite with a micritic structure. This dolomite mostly exhibits laminae of 0.1–1 mm thick and is often discovered with other minerals, such as albite, analcite, barite and dickite, which have at least two types of interbedded laminae. Petrogeochemistry reveals that this dolostone contains a large number of typomorphic elements of hydrothermal sedimentation, including Sb, Ba, Sr, Mn, and V. In addition, the LREE is in relatively high concentrations and possesses the typical REE distribution pattern with negative Eu anomaly. Oxygen isotope values (δ18OPDB) range from 5.89‰ to 14.15‰ with an average of 9.69‰. The ratio of 87Sr/86Sr is between 0.711648 and 0.719546, with an average of 0.714718. These data indicate that the depositional environment is a stable, blocked, anoxic low-lying hot brine pool in the bottom of deep lake controlled by basement faults. The hydrothermal fluid is the alkaline hot brine formed by the combination of the infiltration lake water and mantle-derived magmatic water, consisting of many ions, including Ca2+, Mg2+ and Fe2+. Under the driving flow power of magmatic heat, gravity and compaction, the hydrothermal fluid overcame the overburden pressure and hydrostatic pressure of the lake water body, and boiled to explosion, and then the explosion shattered the original laminated micritic ferruginous primary dolostone near the vent and then formed a new type of dolostone called shattered “hydroexplosion breccias”. In the low-lying, unperturbed hot brine pool, far from the vent, the laminated micritic ferruginous primary dolostone was quickly crystallized and chemicals precipitated from the hydrotherm. This study of special rocks contributes to research into the causes of the formation of lacustrine carbonate rocks and dolostone. In particular, it provides new examples and research insights for future studies of the lacustrine dolomite from the similar Mesozoic and Cenozoic basins in China.

Petroleum systems in the offshore Xihu Basin on the continental shelf of the East China Sea

Xihu Basin is one of the Cenozoic sedimentary basins within the continental shelf of the East China Sea, within which eight oil and gas fields and four hydrocarbon-bearing structures have been found. Our systematic analysis of potential petroleum systems in the basin has identified the Eocene Pinghu Formation as the most important source rock in the basin. The Eocene Pinghu Formation consists of mudstone and coal deposited in an embayment and tidal-flat environment and is characterized as containing type III kerogen. The Pinghu Formation is mature and, at the present time, is within the oil and wet-gas windows with determined vitrinite reflectance values in the range of 0.55–2.2% measured vitrinite reflectance (Rom). Modeling results suggest that the main stage of hydrocarbon expulsion occurred during the Miocene. The main reservoir consists of fine-grained sandstones of the Huagang and Pinghu formations deposited within shallow lacustrine and estuarine environments. The measured porosity from core samples of reservoir rock ranges from 10 to 35%, whereas permeability values range from 1 to 4000 md; both porosity and permeability decrease with depth. The mudstones of the upper Huagang and Longjing formations also occur as regional seals, which developed overpressure zones as determined by two-way sonic transit times. The overburden includes all the Oligocene, Miocene, Pliocene, and Quaternary strata. The hydrocarbon traps in the basin are mainly structural traps, including faulted blocks, faulted structural noses, and anticlines. Most traps were developed at the ends of the Eocene or Oligocene. Hydrocarbons produced from the Xihu Basin are predominantly natural gases with minor amounts of crude oil. The crude oil in the basin is characterized by a low density, low sulfur, low wax, low vanadium/nickel ratio, a low resin and asphaltene content, and a high proportion of saturated hydrocarbons. The natural gas in the basin is composed predominantly of methane, with an average C1/C1–5 ratio of 0.89 and a C1/C2–5 ratio of 8.6. Two petroleum systems have been identified in the basin: the known Pinghu-Huagang and the hypothetical Huagang-Huagang systems. The Pinghu-Huagang is the major petroleum system and most important for oil and natural gas exploration in the basin.

Early dolomitization and recrystallization of carbonate in an evaporite basin: the Middle Devonian Ratner laminite in southern Saskatchewan, Canada

This study provides an example of early dolomitization of basinal limestone associated with a giant evaporite deposit. The Middle Devonian Ratner carbonate is extensively to completely replaced by microcrystalline to finely crystalline dolomite in the southern and central part of southern Saskatchewan in the Elk Point Basin. In the northern part, however, individual cored intervals of the Ratner laminite are either completely dolomitized or contain no microcrystalline to finely crystalline dolomite. Stratigraphic, petrographical and Sr-isotopic data suggest that the Ratner dolomite has formed penecontemporaneously from dense, evaporitic seawater that percolated downward. The distribution of the Ratner dolomite is highly localized in the northern part of the study area, suggesting that the downward flow of Mg2+-rich fluids was restricted by interbedded anhydrite, but where the fluids penetrated, the underlying carbonate deposits were dolomitized. The depletion in 18O isotope (varying from −4.5 to −6.6‰ PDB), and the coarser and inhomogeneous textures of the Ratner dolomite are considered to be the result of recrystallization, which is interpreted to occur at elevated temperatures related to a thermal anomaly during the Late Devonian and Carboniferous burial.

Sedimentology and Reservoir Characteristics of a Middle Jurassic Fluvial System, Datong Basin, Northern China

ABSTRACT An outcrop of the Middle Jurassic Yungang Formation in Datong Basin of northern China was studied in order to provide a better understanding of the architecture and heterogeneity of hydrocarbon reservoirs associated with braided-fluvial deposits. The main section of outcrop, which is continuously exposed over 350 m, is interpreted to have been deposited in the middle-stream reaches of a sandy braided river in a proximal area at the eastern edge of the basin. Seventeen lithofacies were identified, based on petrology and sedimentary structures. These lithofacies were grouped into seven architectural elements, including channel fill, transverse bar, longitudinal bar, diagonal bar, abandoned channel fill, levee, and overbank fines. These sediments were deposited in three stages (active, abandonment, and reactivation), which are related to sediment channel switching and waxing and waning of the fluvial system. The sandstones are mainly arkoses or lithic arkoses, with high matrix content, low compositional and textural maturity, and a variety of cements. A total of 282 specimens were collected from outcrops and analyzed for porosity and permeability. The results show that sandstones from longitudinal bar and channel fills have the highest porosity and permeability while abandoned channels and levee sediments have the lowest. In cross-section, porosity and permeability are generally highest in the middle of the sand bodies, reflecting the spatial distribution of architectural elements and petrophysical properties. End_Page 105------------------------

Video cathodoluminescence microscopy of diagenetic cements and its applications

Abstract Diagenetic studies of sedimentary rocks using cold cathodoluminescence (CL) microscopy have been severely limited because of the very low intensity of visible luminescent emissions, which required long photographic exposure times, and because of the difficulty in obtaining quantitative data from CL observations. The solution to this problem is to fit the microscope with a high-sensitivity digital colour video camera linked to a computer with image enhancement and image analysis software. The new technique described in this paper: • produces digital CL images of consistent high quality, both quickly and cheaply; • controls the capture and editing of CL images, to reveal detailed textural information even from minerals exhibiting low level luminescence such as quartz; • acquires quantitative information on pore systems and the abundance of cement zones from CL images. Examples are presented to demonstrate the high quality of images produced in this way and the range of uses to which the new technique can be applied. The ability to image exactly the same field of view in both plane polarised light and CL is a particular advantage. Image analysis techniques have been developed to give a statistical characterisation of both pore systems and cement phases that infill them. The abundance of contrasting cement zones seen in CL can be measured and the abundance of cement phases can be mapped across a reservoir. The history of porosity occlusion can be reconstructed quantitatively and integrated with burial history and hydrocarbon migration. Porosity can be measured accurately and, since the technique obtains data on pore geometry as well as abundance, the pore system can be characterised by pore size distributions and pseudo-capillary pressure curves. This also opens the possibility of estimating permeability from thin sections.

An overpressured fluid system associated with the giant sandstone-hosted Jinding Zn-Pb deposit, western Yunnan, China

The Jinding Zn-Pb deposit is hosted in continental clastic rocks in the Meso-Cenozoic Lanping-Simao Basin in western Yunnan. The deposit has been compared to SEDEX, MVT, and sandstone-hosted types (SST), all assuming intra-basinal origins for the ore-forming fluids. The driving forces of fluid flow and the pressure system have not been systematically investigated. This paper presents field and fluid-inclusion evidence to show that the Jinding mineralizing system was strongly overpressured. The common occurrence of CO2-rich fluid inclusions and the high fluid pressures suggest a mineralizing system markedly different from most other sediment-hosted base metal deposits including SEDEX, MVT and SST. Numerical modeling of basinal fluid flow indicates that sediment compaction alone in the Lanping-Simao Basin cannot produce any significant overpressure. Tectonic thrusting can significantly increase fluid pressure, but is not enough to cause the high fluid overpressure indicated by fluid inclusions. Input of extra-basinal fluids (e.g. deepsourced CO2) into the basin may have contributed to the development of the overpressured mineralizing system.

Pool characterization of Ordovician Midale field: Implication for Red River play in northern Williston basin, southeastern Saskatchewan, Canada

The Upper Ordovician Midale field is located in the northern Williston basin in southeast Saskatchewan, Canada. Hydrocarbons are hosted mainly in the dolomite reservoirs with burrowed textures in the upper Yeoman Formation. These reservoirs are characterized by intercrystalline porosity in the dolomitized matrix, with variable amounts of vugs and fractures, and can be divided into four zones. Reservoir zones 1 and 2, typically 6–10 m (20–33 ft) thick in total, are situated in the upper part of the traps and commonly bear oil. Although the underlying zones 3 and 4 are thicker, they commonly contain only water because they are located below the spillpoint of the hydrocarbon traps.The seismic reflection of the Red River reservoirs in the Midale field is characterized by a weak- to medium-amplitude trough immediately above the positive reflection of the Winnipeg shale. Where all four zones are present, an additional peak occurs on the seismic profile above the original reservoir reflection. This additional peak, however, disappears where reservoir zones 3 and 4 pinch out. Where there is an increase in the thickness of reservoir zones 1 and 2 or amalgamation of zone 1 with zone 2, the Red River reservoirs are characterized by high-amplitude and high-frequency reflections on seismic profiles.The Ordovician oil pools in the Midale area are associated with low-relief anticline structures. These low-relief structures are interpreted as the compactional drape of Red River strata over local Precambrian basement highs. The source of hydrocarbons in the Red River reservoirs is Ordovician kukersites. A wide range of API fractions for the oils from the Midale pools suggests a mixing of low-maturity oils, sourced from local kukersite beds, and high-maturity oils that migrated over a long distance from the south. The hydrocarbon production from Red River Midale pools is characterized by the fast rise of water cut and high water output, which can be attributed to the small pool size and the fracture systems connecting oil and water zones.