Medium-coarse Sandstone Research Articles

Dynamic Change Characteristics and Main Controlling Factors of Pore Gas and Water in Tight Reservoir of Yan’an Gas Field in Ordos Basin

Tight sandstone gas has become an important field of natural gas development in China. The tight sandstone gas resources of Yan’an gas field in Ordos Basin have made great progress. However, due to the complex gas–water relationship, its exploration and development have been seriously restricted. The occurrence state of water molecules in tight reservoirs, the dynamic change characteristics of gas–water two-phase seepage and its main controlling factors are still unclear. In this paper, the water-occurrence state, gas–water two-phase fluid distribution and dynamic change characteristics of different types of tight reservoir rock samples in Yan’an gas field were studied by means of water vapor isothermal adsorption experiment and nuclear magnetic resonance methane flooding experiment, and the main controlling factors were discussed. The results show that water molecules in different types of tight reservoirs mainly occur in clay minerals and their main participation is in the formation of fractured and parallel plate pores. The adsorption characteristics of water molecules conform to the Dent model; that is, the adsorption is divided into single-layer adsorption, multi-layer adsorption and capillary condensation. In mudstone, limestone and fine sandstone, water mainly occurs in small-sized pores with a diameter of 0.001 μm–0.1 μm. The dynamic change characteristics of gas and water are not obvious and no longer change under 7 MPa displacement pressure, and the gas saturation is low. The gas–water dynamic change characteristics of conglomerate and medium-coarse sandstone are obvious and no longer change under 9 MPa displacement pressure. The gas saturation is high, and the water molecules mainly exist in large-sized pores with a diameter of 0.1 μm–10 μm. The development of organic matter in tight reservoir mudstone is not conducive to the occurrence of water molecules. Clay minerals are the main reason for the high water saturation of different types of tight reservoir rocks. Tight rock reservoirs with large pore size and low clay mineral content are more conducive to natural gas migration and occurrence, which is conducive to tight sandstone gas accumulation.

Along-Strike Reservoir Development of Steep-Slope Depositional Systems: Case Study from Liushagang Formation in the Weixinan Sag, Beibuwan Basin, South China Sea

Seismic, core, drilling, logging, and thin-section data are considered to analyze the reservoir diversity in the east, middle, and west fan of the Liushagang Formation in the steep-slope zone of the Weixinan Sag, Beibuwan Basin. Three factors primarily affect the reservoir differences for steep-slope systems: (1) Sedimentary factors mostly control reservoir scales and characteristics and the drainage system and microfacies. Massive high-quality reservoirs have shallow burial depths. Channel development and sediment supply favor the formation of these reservoirs. The sedimentary microfacies suggest fan delta plain distributary channels. (2) Lithofacies factors primarily control reservoir types and evolution. The diagenesis of high-quality reservoirs is weak, and a weak compaction–cementation diagenetic facies and medium compaction–dissolution diagenetic facies were developed. (3) Sandstone thickness factors primarily control the oil-bearing properties of reservoirs. The average porosity and permeability of high-quality reservoirs are large, the critical sandstone thickness is small, the average sandstone thickness is large, and the oil-bearing capacity is high. Furthermore, the reservoir prediction models are summarized as fan delta and nearshore subaqueous fan models. The high-quality reservoir of the fan delta model is in the fan delta plain, and the lithology is medium–coarse sandstone. The organic acid + meteoric freshwater two-stage dissolution is developed, various dissolved pores are formed, and a Type I reservoir is developed. The high-quality reservoir of the nearshore subaqueous fan model is in the middle fan, and the lithology is primarily medium–fine sandstone. Only organic acid dissolution, dissolution pores, and Type I–II reservoirs are developed. Regarding reservoir differences and models, the high-quality reservoir of the steep-slope system is shallow and large-scale, and the reservoir is a fan delta plain distributary channel microfacies. Weak diagenetic evolution, good physical properties, thick sandstone, and good oil-bearing properties developed a Type I reservoir. The study of reservoir control factors of the northern steep-slope zone was undertaken in order to guide high-quality reservoir predictions. Further, it provides a reference for high-quality reservoir distribution and a prediction model for the steep-slope system.

Facies, textural and geochemical evaluation of the post-Santonian sandstones in the Bida basin, Nigeria: Implications for control on hydrocarbon sandstone reservoir characteristics and paleoenvironments

Using a combination of field observations, textural analysis, scanning electron microscopy (SEM), mineralogical X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), and thin-section analysis, the hydrocarbon and paleoenvironmental characteristics of the Cretaceous sandstones of the southern Bida Basin, Nigeria, were evaluated. SEM data were used to display the mineral morphology, degree of cementation and presence of clay minerals on the grain surfaces and their pores on both sandstones of the Patti and Lokoja formations. The lithofacies analyses revealed three distinct facies associations. The XRD data showed that for the Lokoja and Patti sandstone reservoirs contained quartz from 49 to 67% and 43–68%, respectively. The XRF data also showed that both formations' sandstone reservoirs have a high percentage of silicon and low aluminum content. The grain size study showed that the Lokoja sandstone reservoirs include medium-coarse sandstone (−0.63 to 1.30 ф) that are poorly-moderately sorted (0.72–1.82 ф). In contrast, the Patti Formation is primarily made up of fine-medium sandstones at investigated outcrops, which typically display reasonably good to poor sorting (0.70–2.53 ф). The skewness of the sandstone reservoirs ranged from fine to coarse, and their kurtosis ranged from very platykurtic to leptokurtic. The research area's predominant depositional environments include shoreface/foreshore, estuary mudflat, and estuarine delta with fluvial interference, according to an analysis that combined sedimentary facies and textural data. This demonstrates that the sandstones from the two units have moderate quality prospective hydrocarbon reservoirs based on their physical and chemical characteristics.

Application of Full-rotation Complete Tube Technology in the Construction of Pile Foundation in Medium-coarse Sand Geology

<p>The full-rotation complete tube technology is to drive the flower tube and the steel casing to be fastened by connecting pins, drive the steel casing to rotate 360 degrees, and apply torque and vertical load to the steel casing in the hole, with the help of the steel casing pipe to cut and drill the boulder and medium-coarse sandstone layer with strong force, make full use of the role of the steel casing to protect the wall, and then use the auger and sand drill to grab the confined water and the soil in the steel casing, and then clear thelobstacles inside of the steel casing. Controlling the slump of concrete and the amount of concrete poured each time can more effectively control the construction quality, which has obvious effects on speeding up the construction progress and improving efficiency, and at the same time avoiding environmental pollution.</p>

Analysis of lithofacies and evaluation of effective reservoirs of member 2 of Xujiahe Formation in the Xinchang area in Western Sichuan

In the Xinchang area in western Sichuan Basin, a total of 121.12 billion cubic meters of proved reserves of natural gas are discovered in Member 2 of Xujiahe Formation, but only 7.739 billion cubic meters of proved reserves of natural gas are produced. The main reason is that sandstones in Member 2 of Xujiahe Formation in the Xinchang area are generally tight, with strong heterogeneity, only intervals with relatively good physical properties have a certain production capacity; therefore, it is very necessary and important to evaluate effective reservoirs in Member 2 of Xujiahe Formation. Through analysis of cores, with consideration of color, sedimentary texture and grain size, lithofacies of Member 2 of Xujiahe Formation in the Xinchang area can be divided into 12 types. According to data of physical properties and thin section analysis, it can be concluded that the favorable lithofacies in the upper submember of Member 2 of Xujiahe Formation are the massive medium-coarse sandstone, the medium-coarse sandstone with cross bedding, and the medium-coarse sandstone with carbonaceous fragments. On the basis of electrical characteristics, well-logging interpretation standards for favorable lithofacies are established, in addition, favorable lithofacies and effective reservoir in the Xinchang area are determined, and distribution of favorable lithofacies (effective reservoir) is identified, which lay a solid foundation for next deployment of appraisal well and development of effective reserves.

Effect of Water on Long-Term Strength of Column Rocks Based on Creep Behavior in Yungang Grottoes, China

The long-term strength of rock load-bearing elements is an important mechanical index to determine its bearing capacity and preservation when protecting cultural relics made of stone. In this paper, uniaxial compressive creep experiments of medium-coarse sandstone from the columns of the Yungang Grottoes were performed on both dried and water-saturated samples. The effects of rock-water interaction on the long-term strength of rocks and on the bearing capacity to evaluate safety were determined. The experimental results show that the instantaneous strength and long-term strength of the medium-coarse sandstone in a water-saturated state were lower than the dried samples. The creep properties of the saturated rock were more obvious and the time needed to achieve a steady state creep was longer. The safety factors of the long-term bearing capacity of water-saturated rocks were not high enough, which indicates the possibility of unstable damage to the columns. It is recommended to strengthen the column in a timely manner to prevent damage and potential safety issues, particularly waterproofing the columns for addition protection. Finally, these experimental results provide a reference for the long-term strength evaluation of rock columns, and provide a theoretical basis for waterproofing the World Cultural Heritage, Yungang Grottoes.

Depositional system and favorable reservoir distribution of Xujiahe Formation in Sichuan Basin

Abstract By comprehensive analysis on the depositional system and on the main controlling factors of reservoirs in the Upper Triassic Xujiahe Formation in Sichuan Basin, the formation conditions of high-quality reservoirs under low-porosity and low-permeability background were studied. Except the transitional facies of Xu-1 member, other members of Xujiahe Formation are of terrestrial deposit, with alluvial fan, fluvial, fan-delta, fluvial-delta, and lacustrine deposit systems developed. Both the facies belt distribution and paleogeographic evolution are closely related to the regional tectonic movement. The original sedimentary facies belt (sandbar microfacies of main stem of braided river) and later denudation co-control the distribution of favorable reservoirs, and the quartz overgrowth and cementation is one of the controlling factors of the tight sandstones. The favorable reservoirs are mainly developed in the high-energy channel of delta plain, with medium-coarse sandstone as the best lithology. Combined with the analysis data of effective thickness and physical property, the favorable reservoirs in the Xu-2, 4, 6 members were evaluated and predicted comprehensively. The results indicate that the center and south of Sichuan are the most favorable reservoir distribution areas.

Glacier-fed(?) sandstone sheets in the Weller Coal Measures (Permian), Allan Hills, Antarctica

The upper member of the Weller Coal Measures (Permian) at Allan Hills, Victoria Land, Antarctica, consists of repetitive coal layers separated by sheetlike bodies of predominantly sandstone and subordinate siltstone and conglomerate, all of fluvial origin. Investigation of the lowermost inter-coal sheet (WC1) reveals abrupt planar basal contacts, persistent upward fining, and a strong westward palaeocurrent trend. Three lithofacies are recognized in the WC1 sheet: (A) small-scale cross-bedded pebbly sandstone, which occupies most of the basal portions of the sheet, (B) large-scale cross-bedded medium-coarse sandstone, confined to a linear NE–SW belt overlying or locally replacing facies A, and (C) siltstone and rippled fine-grained sandstone, which drapes and overlaps facies A and B and contains Glossopteris leaves and Vertebraria root traces. Facies distribution patterns indicate the WC1 body initiated as an abrupt sheetlike incursion of coarse sediment over a low-lying mire (facies A), followed by entrenchment and confinement of flow as the sheet extended distally (facies B). Facies C represents low-energy floodplain sedimentation co-existing with and later succeeding facies B as deposition evolved toward abandonment and eventual renewal of peat deposition. Relatively rapid deposition of the sandstone sheet is implied by the lack of any significant hiatus markers. Outsized clasts up to 45 cm in diameter, one observed to display prominent striations, occur sporadically throughout the WC1, including within siltstone beds of facies C. These are inferred to be ice-rafted deposits which, together with other evidence, suggest meltwater influence, possibly glacial, on sediment transport and deposition. Preliminary examination of other sandstone sheets in the upper Weller indicates attributes similar to the WC1, including abrupt bases, upward-fining character, and predominantly westward palaeocurrent patterns. The Allan Hills exposures are believed to represent a succession of low-lying mire complexes interrupted by abrupt but infrequent incursions of coarse sediment, possibly by extreme meltwater floods.

Controls on fluvial deposition in mixed fluvial and aeolian facies within the Tumblagooda Sandstone (Late Silurian) of Western Australia

The Tumblagooda Sandstone (? Late Silurian) of Western Australia was deposited dominantly by fluvial processes on a palaeoslope to the northwest. Interbedded fluvial and aeolian sandsheet deposits occur within the formation, in which winds blowing obliquely up the palaeoslope to the southeast reworked the sandy, unvegetated, and unconsolidated surface into aeolian sandsheets and small parallel crested aeolian dunes with NE-SW crest lines. The water table was generally high but flashy river discharge resulted in deposition of thin (up to 3 m) laterally extensive (hundreds to thousands of metres) sheets of medium-coarse sandstone with dominant trough cross-bedding and subordinate planar cross-bedding and parallel laminated beds. In the absence of vegetation and levees, floods were virtually unconfined and current directions are locally unimodal with narrow variation. Flood waters were sometimes diverted by aeolian surface morphology. Similar mixed fluvial and aeolian environments are considered to have been frequent in a wide range of climatic regimes prior to the advent of abundant land plants.

Minimum Conditions for Migration of Oil in Water-Wet Carbonate Rocks

The minimum conditions for migration of oil through water-wet are of great interest to geologists concerned with regional exploration problems. Oil can not migrate either under hydrostatic or hydrodynamic conditions unless the displacement pressure exceeds the entry pressure of the pore interconnections. Since the entry pressure is largely a function of pore size and configuration, thin sections of various carbonate rocks were studied in detail. One hundred pores and corresponding interconnections were measured, and 50 pore-throat configurations were analyzed and classified as to pore-space type. The pore-size measurements ranged from 0.02 mm. to 1.40 mm., with a logarithmic mean of 0.137 mm.. The measurements of the interconnections ranged from 0.0003 mm. to 0.123 mm., w th a logarithmic mean 0.0163 mm. In the pore-space type most frequently observed, the interconnections between pores were constrictions formed by individual carbonate crystals or fragments projecting into the pore space. The logarithmic mean values for pore and throat size were used together with representative average values for crude oil and brine to estimate the minimum conditions for migration of oil through water-wet carbonates. Under hydrostatic conditions, the product of the sine of the regional dip angle (^Thgr) and the length of a continuous oil phase (L) must be greater than the critical height for buoyant rise (h) in order to move oil through water-wet carbonates, i.e., sin ^ThgrL=h, where h was calculated to be 231 cm. (7½ feet). Hence, the displacement pressure needed is 6½ times that required for oil migration through average water-wet medium-coarse sandstone. Under hydrodynamic conditions, the interaction of permeability, porosity, viscosity and flow rate of the water, and oil phase length must produce a critical pressure gradient of 44,105 dynes/cm.2/L cm. in order to move oil through average carbonate rocks. The following are tentative conclusions. 1. The dominant pore-space type influences considerably the mninimum conditions for migration. 2. The average depositional dips of shelf-type carbonates are probably insufficient to cause largescale migration under hydrostatic conditions, unless these dips are associated with hinge lines or are amplified by tectonic disturbances. 3. Even low rates of flow of water can initiate and sustain migration of short continuous oil phases. 4. In basins where both carbonates and sandstones act as carriers and reservoirs and where the dips are generally gentle, the regional concentration of oil prior to localized entrapment occurs farther downdip for the carbonates than for sandstones. 5. Exploration on a regional scale should take into account the difference in the minimum conditions for migration of oil which exist between carbonate reservoir rocks and porous and permeable sandstones.