Search

Mesozoic marine shale oil was found in the Qiangtang Basin by a large number of hydrocarbon geological surveys and shallow drilling sampling. Based on systematic observation and experimental analysis of outcrop and core samples, the deposition and development conditions and characteristics of marine shale are revealed, the geochemical and reservoir characteristics of marine shale are evaluated, and the layers of marine shale oil in the Mesozoic are determined. The following geological understandings are obtained. First, there are two sets of marine organic-rich shales, the Lower Jurassic Quse Formation and the Upper Triassic Bagong Formation, in the Qiangtang Basin. They are mainly composed of laminated shale with massive mudstone. The laminated organic-rich shale of the Quse Formation is located in the lower part of the stratum, with a thickness of 50–75 m, and mainly distributed in southern Qiangtang Basin and the central-west of northern Qiangtang Basin. The laminated organic-rich shale of the Bagong Formation is located in the middle of the stratum, with a thickness of 250–350 m, and distributed in both northern and southern Qiangtang Basin. Second, the two sets of laminated organic-rich shales develop foliation, and various types of micropores and microfractures. The average content of brittle minerals is 70%, implying a high fracturability. The average porosity is 5.89%, indicating good reservoir physical properties to the level of moderate–good shale oil reservoirs. Third, the organic-rich shale of the Quse Formation contains organic matters of types II1 and II2, with the average TOC of 8.34%, the average content of chloroform bitumen ‘A’ of 0.66%, the average residual hydrocarbon generation potential (S1+S2) of 29.93 mg/g, and the Ro value of 0.9%–1.3%, meeting the standard of high-quality source rock. The organic-rich shale of the Bagong Formation contains mixed organic matters, with the TOC of 0.65%–3.10% and the Ro value of 1.17%–1.59%, meeting the standard of moderate source rock. Fourth, four shallow wells (depth of 50–250 m) with oil shows have been found in the organic shales at 50–90 m in the lower part of the Bagong Formation and 30–75 m in the middle part of the Quse Formation. The crude oil contains a high content of saturated hydrocarbon. Analysis and testing of outcrop and shallow well samples confirm the presence of marine shale oil in the Bagong Formation and the Quse Formation. Good shale oil intervals in the Bagong Formation are observed in layers 18–20 in the lower part of the section, where the shales with (S0+S1) higher than 1 mg/g are 206.7 m thick, with the maximum and average (S0+S1) of 1.92 mg/g and 1.81 mg/g, respectively. Good shale oil intervals in the Quse Formation are found in layers 4–8 in the lower part of the section, where the shales with (S0+S1) higher than 1 mg/g are 58.8 m thick, with the maximum and average (S0+S1) of 6.46 mg/g and 2.23 mg/g, respectively.

Usually, deep oil and gas accumulation is often controlled by strike–slip faults. However, in the Tarim Basin, deep Ordovician oil and gas accumulations are also found in areas far from the fault zone. The process of oil and gas accumulation in deep reservoirs far from strike–slip fault zones is still unclear at present. The source and evolution of Ordovician fluids were analyzed using inclusion geochemical methods and the U–Pb dating technique. The analysis of rare earth elements and carbon–oxygen–strontium isotopes in the reservoirs showed that the reservoirs were weakly modified by diagenetic fluid. The fluid was derived from the fluid formation during the same period as the seawater, and no oxidizing fluid invaded the reservoir. The late oil and gas reservoirs had good sealing properties. The U–Pb dating results combined with homogenization temperature data revealed that the first-stage oil was charged during the Late Caledonian Period, and the second-stage natural gas was charged during the Middle Yanshanian Period. The evolution of the paleo-pressure showed that the charging of natural gas in the Middle Yanshanian was the main reason for the formation of reservoir overpressure. The strike–slip fault zone was basically inactive in the Middle Yanshanian. During this period, the charged natural gas mainly migrated to the reservoir along the unconformity surface and the open strike–slip fault zone in the upper part of the Ordovician reservoir. The source of the fluid shows that the reservoir in the late stage had good sealing properties, and there was no intrusion of exogenous fluid. The overpressure in the reservoir is well preserved at present.

Aiming at the scientific problem that only part of dolomite acts as dolomite reservoir, this paper takes the multiple dolomite-bearing formations in the Tarim and Ordos basins, NW China and Sichuan Basin, SW China as the study object, by means of mineral petrological analysis and geochemical methods including carbonate clumped isotope, U-Pb isotopic dating, etc., to rebuild the dolomitization pathway and evaluate its effects on reservoir formation. On the basis of detailed rock thin section observation, five dolomitic structural components are identified, including original fabric-retained dolomite (microbial and/or micrite structure), buried metasomatic dolomite I (subhedral–euhedral fine, medium and coarse crystalline structure), buried metasomatic dolomite II (allotriomorphic–subhedral fine, medium and coarse crystalline structure), buried precipitation dolomite and coarse crystalline saddle dolomite. Among them, the first three exist in the form of rocks, the latter two occur as dolomite minerals filling in pores and fractures. The corresponding petrological and geochemical identification templates for them are established. Based on the identification of the five dolomitic structural components, six dolomitization pathways for three types of reservoirs (preserved dolomite, reworked dolomite and limestone buried dolomitization) are distinguished. The initial porosity of the original rock before dolomitization and the dolomitization pathway are the main factors controlling the development of dolomite reservoirs. The preserved dolomite and reworked dolomite types have the most favorable dolomitization pathway for reservoir formation, and are large scale and controlled by sedimentary facies in development and distribution, making them the first choices for oil and gas exploration in deep carbonate formations.

The permeability interpretation accuracy of single wells in the Middle East super-thick carbonate reservoir can hardly meet the actual demand of water injection development. Therefore, this study develops an iterative split algorithm to calculate single-well segmental dynamic permeability and SPI (specific production index) based on the dynamic and static well test, PLT (production log test), production test, and logging interpretation data. Moreover, the dynamic permeability classification evaluation index was optimized. The detailed steps are as follows: 1) The liquid supply of a single well is split according to tp PLT. 2) For some production intervals, the log interpretation data are applied to carry out the second iteration splitting of formation coefficient and fluid supply. 3) The proportion of liquid supply in the stratification section is calculated based on the calculation results of steps 1 and step 2. 4) The total formation coefficient of well test interpretation is divided based on the calculation results of step 3. 5) The dynamic permeability and SPI are calculated. The research results show that the dynamic permeability and SPI of a single well are accurate to the scale of one meter from the several original meters to tens of meters, which can better meet the needs of water injection development. In addition, SPI can be used as an essential index for permeability classification evaluation. Taking the Halfaya Oilfield and the Mishrif format reservoir as examples, the low penetration rate of the MB1-2 high penetration rate is 200md and the lower specific production index limit is 0.35 bbl/(d·m·psi), respectively. The dynamic foundation of the high-permeability layer is identified. This method offers a new idea for the title of high-permeability streaks. Moreover, this method can provide the scientific basis for correcting the permeability attribute model, water injection plan optimization, single-well completion cementing scheme, and water coning control, which can be popularized and applied in the same reservoirs.

To further ascertain the origin of the Ordovician Majiagou Formation reservoirs in the Ordos Basin, the M54-M51 sub-members of the Ordovician Majiagou Formation in the eastern Sulige gasfield of Ordos Basin were taken as examples to analyze the vertical development characteristics of eogenetic karst and to discover the dissolution mechanism and its control on reservoirs through observation of a large number of cores and thin sections. According to detailed analysis of petrologic characteristics, the reservoir rock types include micritic dolomite, grain dolomite and microbialite which have mainly moldic pore, intergranular (dissolved) pore, and (dissolved) residual framework pore as main reservoir space respectively. The study area developed upward-shallowing sequences, with an exposure surface at the top of a single upward-shallowing sequence. The karst systems under the exposure surface had typical exposure characteristics of early dissolution and filling, indicating these reservoirs were related to the facies-controlled eogenetic karstification. With the increase of karstification intensity, the reservoirs became worse in physical properties.

The Zeeman splitting effect is observed in a strong magnetic field generated by a laser-driven coil. The expanding plasma from the coil wire surface is concentrated at the coil center and interacts with the simultaneously generated magnetic field. The Cu I spectral lines at wavelengths of 510.5541, 515.3235, and 521.8202 nm are detected and analyzed. The splittings of spectral lines are used to estimate the magnetic field strength at the coil center as ∼31.4 ± 15.7 T at a laser intensity of ∼5.6 × 1015 W/cm2, which agrees well with measurements using a B-dot probe. Some other plasma parameters of the central plasma disk are also studied. The temperature is evaluated from the Cu I spectral line intensity ratio, while the electron density is estimated from the Stark broadening effect.

Based on comprehensive analysis of core, cast thin section, logging and seismic data, the sedimentary and reservoir architectures of the MB1-2 sub-member of Mishrif Formation in Halfaya Oilfield, Iraq, are studied. The MB1-2 sub-member of Mishrif Formation has three types of microfacies, lagoon, bioclastic shoal, and tidal channel, and facies architecture controlled by sequence stratigraphy. In the 4th-order sequence, the lagoon facies aggradated vertically, and the bioclastic shoals in lenticular shape embed in the background of lagoon, the end of the sequence is incised by the “meandering river” shape tide channel, which represents the depositional discontinuity. Three types of reservoirs including tidal channel grainstone to packstone reservoirs, bioclastic shoal grainstone to packstone reservoirs and dissolved lagoon wackestone reservoirs are developed. The reservoir architectures within tidal channel and bioclastic shoal are strickly controlled by grainy facies, whereas the dissolved lagoon reservoirs controlled by both facies and dissolution are related to the sequence boundary. The reservoir sections occur mainly in the 4th sequence highstand systems tract (HST) and are separated by barriers formed in the transgressive systems tract (TST). Complicated facies architecture and dissolution modification resulted in strong heterogeneity within the reservoir, which showed the characteristics of “attic type” architecture. The results of this study can guide the development of similar reservoirs in the Middle East.

Abstract: Taking the Cambrian Yuertus Formation outcrop profiles in the Aksu-Keping-Wushi areas of northwestern Tarim Basin as examples, the depositional environments of organic rich fine sediment were analyzed by examining the outcrop profiles macroscopically and microscopically. The study reveals that: (1) The lower part of the Yuertus Formation consists of organic-rich fine sediment or thin rhythmic interbeds of organic-rich fine sediment and siliceous sediment, the formation transforms to terrigenous diamictic grain shoal and inverse grading carbonate rocks upward. (2) The thin limestone interbedded with dark shale rhythmically has inverse grading. (3) The thin-bedded siliceous rock has metasomatic residual granular texture, stromatolithic structure and cementation fabric in vugs. (4) There are iron crust layers at the top of the shallowing diamictic grain shoal, beneath which exposed karst signs, such as karrens, dissolved fissures, sack-like vugs, near surface karst (plastic) breccia, breccia inside the karst system and terrigenous clastic fillings, can be seen. (5) Both the outcrops and seismic profiles show that organic-rich fine sediments above the unconformities or exposed surfaces are characterized by overlapping. The organic-rich fine sediment of the Cambrian Yuertus Formation was deposited in the anoxic-suboxidized restricted gulf lagoon environment, and its formation was controlled by high paleoproductivity and poor oxygen exchange jointly, then a shallow-water overlapping sedimentary model has been established. The results will help enrich and improve the sedimentary theory of organic-rich fine sediments.

AbstractThe carbonate ramps developed at the postrift depositional stage have become a major field of ultradeep hydrocarbon exploration. Taking the Lower Cambrian Xiaoerblak Formation in the Tarim Basin as an example, the paleotectonic framework and lithofacies paleogeography response, reservoir distribution, and internal architecture difference were systematically analysed based on the data of newly processed 42 2D seismic lines, 9 outcrops, and 22 wells. The results show that (1) the palaeotectonic framework with three palaeohighs and two topographic lows (abbreviated to “TPTT”) dominated the differentiation in the ramp system (association) at the Xiaoerblak depositional stage. High-energy grain shoals and mounds occurred around the palaeohighs, and the sediment composition was further complicated by ancient ocean currents. Three subtypes of ramp are recognized: the slope-plateau ramp dominated by grain shoal on the northern margin of the paleohigh of the southwestern Tarim, homocline ramp dominated by the mound-shoal complex on the Keping-Wensu area lower high, and isolated island type in the Lunnan-Yaha lower high. (2) Xiaoerblak reservoirs mainly occur in the high-energy facies belts in the middle ramps around the palaeohighs because original pores were effectively preserved in early dolomitization and reconstructed at the burial stage. Reservoirs are predicted to mainly distribute in the high-energy mound-shoal zones around three palaeohighs (low uplifts) with an area of 9×104 km2. Different ramp systems show different reservoir properties. There are mainly type II reservoirs (porosity range of 1.8-4.5%, permeability range of 0.5-10×10−3 μm2) with sustained distribution and the largest size in the grain shoals at the slope-plateau ramp, which may be the potential favorable hydrocarbon exploration zone. Based on the example anatomy, the differentiation in the carbonate ramp systems forming at the postrift subsidence stage was structurally dominated. This means that it is crucial to deepen the understanding of the presedimentation palaeotectonic framework for the prediction of promising reservoirs and assessment of reservoir properties.

The microbial dolomite of the Ediacaran Dengying Fm., Sichuan Basin, SW China is endowed with abundant oil and gas resources. A complex diagenetic history greatly complicates the development of such microbial dolomite reservoirs, and has severely restrained hydrocarbon exploration. This study focused on key field profiles in Eastern Sichuan and aimed to decipher the origin and evolution of dolomites, using novel techniques, including elemental mapping, laser ablation U-Pb dating as well as carbon, oxygen, and strontium isotope analyses. Additionally, we compared the diagenesis and reservoir development model for the Dengying microbial dolomites of the Central Sichuan to other areas, with the aim to provide practical guidance for oil and gas exploration across the entire Sichuan Basin. Our results have shown that the Ediacaran microbial dolomite in Eastern Sichuan experienced four stages of diagenetic modification–specifically: (1) syn-sedimentary dolomitization; (2) penecontemporaneous cementation of fibrous dolomite cement; (3) bladed dolomite and brown-dark very fine crystalline dolomite cementation during the Ordovician; and (4) fine crystalline dolomite and medium-coarse crystalline dolomite cementation during the Silurian-Devonian. Petrology and geochemistry data imply that each diagenetic phase of dolomitization was mainly sourced from marine-dominated fluids. The influence of tectonically related fluids was rarely noted. Significantly, U-Pb ages of these dolomites reveal that the history of dolomite infill to pores within the Eastern Sichuan Basin clearly post-dated that in the Central Sichuan region, resulting in high porosities (6%–11%) that favored hydrocarbon accumulation. This study suggests that the platform margin at the eastern edge of the Xuanhan-Kaijiang paleo-uplift is favorable as the next key exploration target for microbial dolomites in the Sichuan Basin.

AbstractSuperimposed tectonic movement and karst erosion resulted in a combination of fractures and irregular caves in deep/ultradeep carbonate rocks, typically along major fault swarms. Outlining these fault-karst reservoirs mainly depends on recognizing the strong reflection in seismic profiles; however, characterizing their internal structures is still difficult, which are represented as weak amplitude in seismic profiles. This study intended to propose a method to dissect the internal structure of fault-karst reservoirs, which contains four steps: (1) elimination of the signal interference by the covering bed with strong energy and recognition of internal reservoirs with low energy based on seismic data conversion, frequency division, and inversion; (2) gradient structure tensor analysis based on an anisotropic Gaussian filter for fault-karst reservoir outlining; (3) internal faults and cave recognition on the basis of wave-based inversion; and (4) reevaluation of the number and scale of these faults and caves based on seismic recognition and well test results and verification of their volumes and hydrocarbon reserves. The method was used in the evaluation of the fault-karst reservoir in the Halahatang (HLHT) oilfield, which is located in the north of Tarim Basin. The results show that the fault-karst reservoirs along major faults and their internal structures are effectively recognized, and the error of the predicted depth of the reservoirs decreases from more than 20 m before to less than 4 m now; the drilling success ratio increases from 70% to 90%. In addition, the method supports the recognition of untapped fault-karst reservoirs around shut-in wells, which provides guidance for sidetracking plans. Further, by comparing the geophysical volume of fault-karst reservoirs and the reserve predicted by production performance, the untapped reserve in a certain reservoir can be evaluated; on this basis, producing wells received high yields by targeted acid fracturing. In summary, the method effectively improves the prediction accuracy and the recovery efficiency of fault-karst reservoirs.

To find out the origin of dolomite, the precipitation of primary dolomite, and the formation of pores in dolomite, petrologic and geochemical characteristics of typical samples from Sichuan and Tarim Basin were analyzed based on the previous understandings, and three aspects of results were achieved. (1) A classification of dolomite origins based on petrologic features, forming environment, and time sequence was proposed, which shows clear boundaries of diagenetic and characteristic realms and evolved clues between different types of dolomite. (2) Petrographic and geochemical identification marks for different types of dolomite were presented, revealing that the orderly geochemical variation of different types of dolomite is the response to the change of forming environment of dolomite during continuous time sequence. (3) The contribution of dolomitization to the formation of porosity was re-evaluated, revealing that the porosity in dolomite was mostly attributed to the primary pores and supergene dissolution and burial dissolution, and early dolomitization was conducive to the preservation of primary pores. These understandings are of great theoretical significance for identifying the origins and types of dolomite, and can guide the prediction of dolomite reservoirs. Key words: dolomite genetic types, dolomite distinguished characteristics, dolomite porosity origin, protodolomite, primary porosity

To better know the spatial distribution and architecture of carbonate reservoirs, three-dimensional carbonate reservoir geologic modeling based on the digital outcrop model (DOM) is proposed. Based on the traditional geologic study of outcrops, combined with digitizing the outcrop walls by utilizing the advanced instrument (LIDAR, RTK-GPS, GPR, Gigapan, etc), DOM is built, from which geological information based on measured sections and samples (litho-facies, porosity, permeability, sonic velocity) is extracted and used to build the 3-D outcrop reservoir geologic model by modeling software. Eventually the 3-D reservoir geologic model of outcrop is used to guide the subsurface research. The DOM-based 3-D reservoir geologic model for oolitic reservoirs of Triassic Feixianguan Formation in Yudongliang outcrop, NW Sichuan Basin, reveals more realistic spatial distribution of litho-facies, porosity and permeability, and their relationship, consequently providing more reliable evidence for seismic data interpretation and reservoir prediction of subsurface reservoirs with similar geological conditions. Key words: digital outcrop model, carbonate reservoir, 3-D geologic modeling, Triassic Feixianguan Formation, oolitic beach

The driving mechanism of solar flares and coronal mass ejections is a topic of ongoing debate, apart from the consensus that magnetic reconnection plays a key role during the impulsive process. While present solar research mostly depends on observations and theoretical models, laboratory experiments based on high-energy density facilities provide the third method for quantitatively comparing astrophysical observations and models with data achieved in experimental settings. In this article, we show laboratory modeling of solar flares and coronal mass ejections by constructing the magnetic reconnection system with two mutually approaching laser-produced plasmas circumfused of self-generated megagauss magnetic fields. Due to the Euler similarity between the laboratory and solar plasma systems, the present experiments demonstrate the morphological reproduction of flares and coronal mass ejections in solar observations in a scaled sense, and confirm the theory and model predictions about the current-sheet-born anomalous plasmoid as the initial stage of coronal mass ejections, and the behavior of moving-away plasmoid stretching the primary reconnected field lines into a secondary current sheet conjoined with two bright ridges identified as solar flares.