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Continental shale strata in China are rich in petroleum resources and are an important area to strengthen domestic oil exploration and development. Based on the latest progress in geological research and exploration and development of petroleum inside continental source kitchens of China National Petroleum Corporation exploration areas in recent two years, we have achieved the following results: (1) The geological connotations of continental hydrocarbon accumulation inside source kitchen and “sweet spot” have been proposed. The intra-source petroleum accumulation refers to the accumulation of liquid-rich hydrocarbons retained or captured in the continental organic-rich shale strata, and “sweet spot” refers to the favorable reservoir with higher oil content, better physical properties, easier to stimulate and higher in commercial development value in the overall oil-bearing continental source rock series, they can be divided into three types, interlayer, hybrid sediment and shale. (2) High-quality shale formations in both salt water and freshwater lacustrine basins can generate hydrocarbons on a large scale, shale strata have multiple types of favorable reservoirs with large-scale storage capacity, the intra-source shale strata are overall oil-bearing and large in resource scale, and there are multiple favorable shale series for development. The exploration and development practice is propelling the formation of a series of exploration and development key technologies with “sweet spot exploration” and “volume development” as the core. Some pilot tests of these technologies have provided an important scientific basis for the economic and effective development of hydrocarbon accumulation inside source kitchen, and popularization of these technologies have achieved encouraging results preliminarily. (3) Two types of continental intra-source petroleum resources in China have great potential, including medium-high maturity with liquid-rich hydrocarbons and medium-low maturity with organic-rich matter. The Ordos, Songliao, Bohai Bay and Junggar basins are the main areas of these resources. By addressing the theoretical and technical challenges in the exploration and development, the two types of resources inside continental source kitchens will become the realistic and major strategic replacement oil resources respectively in the future.
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The differences in organic matter abundance and rock composition between shale and mudstone determine the discrepancy of their contributions to the formation of conventional and shale oil/gas reservoirs. The evaluation criteria of source rocks are different in the future exploration in self-sourced petroleum systems. Shales are deposited in deep/semi-deep lacustrine, with low sedimentation rate and chemical depositions of various degrees, while mudstones are mostly formed in shallow lacustrine/lakeside, with high deposition rate and density flow characteristics. Three factors contribute to the enrichment of organic matter in shales, including the "fertility effect" caused by volcanic ash deposition and hydrothermal injection, excessive and over-speed growth of organisms promoted by radioactive materials, and deep-water anaerobic environment and low sedimentation rate to protect the accumulation of organic matter from dilution. Lamellations in shales are easy to be stripped into storage space, and acid water produced during hydrocarbon generation can dissolve some particles to generate new pores. The massive mudstones with high clay content are of poor matrix porosity. Shales with high total organic carbon, developed laminations, relatively good reservoir property, and high brittle mineral content, are the most favorable lithofacies for shale oil exploration and development. It is necessary to conduct investigation on the differences between shale and mudstone reservoirs, to identify resources distribution in shale and mudstone formations, determine the type and standard of “sweet-spot” evaluation parameters, optimize “sweet-spot areas/ sections”, and adopt effective development technologies, which is of great significance to objectively evaluate the total amount and economy of shale oil resources, as well as the scale of effective exploitation.
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Based on the abundant outcrops and 3D seismic data in Yuanba and its surrounding areas in northeastern Sichuan Basin, the characteristics, main controlling factors and development regularities of reservoirs in the fourth member of Sinian Dengying Formation (shortened as Deng 4 Member) were analyzed comprehensively. The results show that the reservoirs were mainly developed in microbial dolomite associated with the microbial mound and dolarenite associated with grain beach, and had intergranular dissolution pores, inter-frame dissolution pores and karst caves as main storage space. The reservoirs were large in scale and strong in heterogeneity. The analysis of the reservoir genetic mechanisms shows that the reservoirs in the fourth member of Dengying Formation were the products of karstification on mound-beach facies deposited in the second episode of Tongwan movement. The karstification had the marks of eogenetic karstification such as granophyric karst system and half-dissociation zone. The reservoirs were controlled by the sedimentary facies belt, the mound beach facies was the decisive factor in reservoir development. Further analysis indicates that the Yuanba area had a geological background of well-developed platform margin mound beach facies, so mound beach bodies can be taken as a favorable exploration target.
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Deep shale layer in the Lower Silurian Longmaxi Formation, southern Sichuan Basin is the major replacement target of shale gas exploration in China. However, the prediction of “sweet-spots” in deep shale gas reservoirs lacks physical basis due to the short of systematic experimental research on the physical properties of the deep shale. Based on petrological, acoustic and hardness measurements, variation law and control factors of dynamic and static elastic properties of the deep shale samples are investigated. The study results show that the deep shale samples are similar to the middle-shallow shale in terms of mineral composition and pore type. Geochemical characteristics of organic-rich shale samples (TOC > 2%) indicate that these shale samples have a framework of microcrystalline quartz grains; the intergranular pores in these shale samples are between rigid quartz grains and have mechanical property of hard pore. The lean-organic shale samples (TOC < 2%), with quartz primarily coming from terrigenous debris, feature plastic clay mineral particles as the support frame in rock texture. Intergranular pores in these samples are between clay particles, and show features of soft pores in mechanical property. The difference in microtexture of the deep shale samples results in an asymmetrical inverted V-type change in velocity with quartz content, and the organic-rich shale samples have a smaller variation rate in velocity-porosity and velocity-organic matter content. Also due to the difference in microtexture, the organic-rich shale and organic-lean shale can be clearly discriminated in the cross plots of P-wave impedance versus Poisson’s ratio as well as elasticity modulus versus Poisson’s ratio. The shale samples with quartz mainly coming from biogenic silica show higher hardness and brittleness, while the shale samples with quartz from terrigenous debris have hardness and brittleness less affected by quartz content. The study results can provide a basis for well-logging interpretation and “sweet spot” prediction of Longmaxi Formation shale gas reservoirs.
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By using core, logging curves, and experiment data, favorable lithofacies types in the 2nd Member of Triassic Xujiahe Formation in the Xinchang area, Sichuan Basin were classified, standard of the favorable lithofacies was established, planar distribution regularities of the favorable lithofacies were identified, and forming mechanisms of the favorable lithofacies and their control effect on production were examined. (1) The 2nd Member of Xujiahe Formation has twelve types of lithofacies, among which multiple layer medium-coarse grain sandstone lithofacies, parallel bedding medium-coarse grain sandstone lithofacies, massive bedding medium-coarse grain sandstone lithofacies, inclined bedding medium-coarse grain sandstone lithofacies, and charcoal-bearing medium-coarse grain sandstone lithofacies with better physical properties and higher gas content are favorable lithofacies; they feature low gamma, low neutron porosity, low resistivity, and high acoustic travel time on logging curves. (2) The sedimentary process controls spatial distribution of sand bodies which are the material basis of the favorable lithofacies; post diagenetic fluids would differentially reconstruct the favorable lithofacies; tectonic activities and abnormal formation pressure made strata slide along the weakness plane, giving rise to fractures in different types of rocks, which can enhance the reservoir permeability significantly. (3) The development degree of favorable lithofacies is a major factor affecting stable production of gas well.
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Based on seismic data, outcrop evidence, logging data and regional aeromagnetic data, the distribution of Nanhua-Sinian rifts in the southwestern Tarim Basin was analyzed, and on the basis of restoration of lithofacies paleogeography in different periods of Neoproterozoic-Cambrian, the evolution model of the proto-type rift basin was discussed. The Neoproterozoic Rodinia supercontinent split event formed the trigeminal rift system at the edge and inside of the craton in the southwestern Tarim Basin located in the Kunlun piedmont and Maigaiti slope. The rift in Kunlun piedmont zone was distributed along the E-W direction and was the oceanic rift in the trigeminal rift system. Two decadent rifts in N-E strike developed in the Luonan and Yubei areas of Maigaiti slope, and the interior of the rifts were characterized by a composite graben-horst structure composed of multiple grabens and horsts. The Neoproterozoic-Cambrian proto-type basin evolution in the southwestern Tarim Basin can be divided into three stages: rift in the Nanhua, embryonic passive continental margin in the Sinian, and stable passive continental margin in the Cambrian. Despite the regional tectonic movements in the end of Nanhua and Sinian, the tectonic framework of the southwestern Tarim Basin had not changed significantly, the sedimentary center of Nanhua rift basin showed the characteristics of succession in the Sinian-Early Cambrian. The Nanhua rift in Kunlun piedmont evolved into a craton marginal depression during the Sinian-Early Paleozoic, while the Luonan decadent rift in the midsection of Maigaiti slope evolved into a sag inside platform in Early Cambrian, constituting the paleogeographic framework of “two paleouplifts and one sag” with the paleouplifts in east and west sides of the slope. The later evolution of the Luonan decadent rift in the midsection of the Maigaiti slope formed two sets of reservoir-forming assemblages, the Sinian and the Lower Cambrian ones, which are important exploration targets in future.
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The characteristics, formation time, and origin of the sucrosic dolomite reservoirs in the Permian Qixia Formation of northwestern Sichuan Basin are analyzed. Core and outcrop description and microscopic analysis of the sucrosic dolomite samples are carried out. It is found that the dolomite has typical features different from other kinds of dolomites: (1) This dolomite is generally medium-coarse in crystal size, and often associated with very finely to finely crystalline dolomite and cave-filling dolomite. (2) Typical identification marks of eogenetic karstification are developed at the top of the upward-shallowing sequence. (3) The medium-coarse crystalline sucrosic dolomite is cut by the early diagenetic karst fabric, and is characterized by dolomite with dissolution edge, dolomite vadose silt in pores, and transgression clay filling between the medium-coarse dolomite crystals. The medium-coarse crystalline sucrosic dolomite was formed earlier than the eogenetic karstification. The sucrosic dolomite with occasional cloudy core and clear rim has bright cathodoluminescence, high inclusions temperature, significant negative skewness carbon and oxygen isotopic compositions, and rare-earth element (REE) pattern similar to seawater, indicating it experienced two periods of dolomitization, evaporative concentration reflux-infiltration and penecontemporaneous seawater circulation hydrothermal fluid dolomitization. The study results not only update the understanding on the dolomitization time of Qixia Formation, demonstrate that the sucrosic dolomite can be formed in the penecontemporaneous stage when seawater reflux superimposed with hydrothermal fluid effects, but also show that the taphorogenesis in the Dongwu period began in the Early Permian. Moreover, the dolomite controlled by the grain bank migration and terrain in the slope break appears in bands of large scale, this knowledge provides basis for expanding the exploration field of this type of reservoirs.
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Under the condition of thin interbeds with great lateral changes in terrestrial basins, a seismic meme inversion method is established based on the analysis of seismic sedimentology technology. The relationship between seismic waveform and high-frequency well logs is established through dynamic clustering of seismic waveform to improve the vertical and horizontal resolution of inversion results; meanwhile, by constructing the Bayesian inversion framework of different seismic facies, the real facies controlled inversion is realized. The forward model verification results show that the seismic meme inversion can realize precise prediction of 3 m thick thin interbeds, proving the rationality and high precision of the method. The application in the Daqing placanticline shows that the seismic meme inversion could identify 2 m thin interbeds, and the coincidence rates of inversion results and drilling data were more than 80%. The seismic meme inversion method can improve the accuracy of reservoir prediction and provides a useful mean for thin interbeds prediction in terrestrial basins.
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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.
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Drilling, seismic and logging data were used to evaluate the hydrocarbon accumulation conditions of the mound-shoal complexes in the platform margin of the fourth member of Sinian Dengying Formation in the east side of the Mianzhu-Changning intracratonic rift in the Sichuan Basin. The four understandings are: (1) The platform margin belt of the Deng 4 Member can be divided into three sections, northern, middle and southern; the middle section is at the core of the Gaoshiti-Moxi paleouplift and the structural high now, while the southern and northern sections are at the slope of the paleouplift and the structural lows now; the three sections have similar development characteristics and reservoir features of platform margin mound-shoal complex. (2) In the margin of the east side of the rift, there are several faults nearly perpendicular to the platform margin belt, the faults divide the platform margin belt into rugged paleo-landform, and the high part developed platform margin mound-shoal complexes and the reservoirs are good in physical properties, while the low part developed inter-beach depression and no mound-shoal complexes, where the reservoirs are poor in physical properties. (3) The six groups of faults nearly perpendicular to the platform margin belt divide the platform margin belt into seven large mound-shoal complexes which have similar hydrocarbon accumulation conditions and accumulation evolution process and are rich in petroleum. (4) The inter shoal depressions between the mound-shoal complexes are characterized by tighter lithology, which can block the updip direction of the mounds and shoals at the lower part of the slope of the paleouplift and are favorable for the later preservation of mound-shoal gas reservoirs. This has been proved by Well Jiaotan 1 and Heshen 2 drilled successfully. The mound-shoal complexes on the platform margin of the structural slope area have a good exploration prospect.
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A series of theoretical explorations and field tests have been carried out to efficiently develop the Mahu tight conglomerate oilfield in the Junggar Basin. Concepts of steered-by-edge fracturing and proactive fracturing interference were proposed. A series of innovative technologies were developed and implemented including optimization of 3-D staggered well pattern, proactive control and utilization of spatial stress field, and synergetic integration of multiple elements. Different from shale, the unique rock fabric and strong heterogeneities of tight conglomerate formation are favorable factors for forming complex fractures, small space well pattern can proactively control and make use of interwell interference to increase the complexity of fracture network, and the “optimum-size and distribution” hydraulic fracturing can be achieved through synergetic optimization. During pilot phase of this field, both depletion with hydraulically fractured vertical wells and volume fracturing in horizontal wells were tested after water injection through vertical wells, then the multi-stage fracturing with horizontal well was taken as the primary development technology. A series of engineering methods were tested, and key development parameters were evaluated such as well spacing, lateral length, fractures spacing, fracturing size, and fracturing operation process. According to geoengineering approach, the 100 m/150 m tridimensional tight-spacing staggered development method was established with systematic integration of big well clusters, multiple stacked pay zones, small well spacing, long lateral length, fine perforation clustering, zipper fracturing and factory operation. According to half-year production performance, 100 m/150 m small spacing wells outperformed 500 m/400 m/300 m spacing wells. Its average estimated ultimate recovery (EUR) of wells was identical with those best wells from large-spacing area. Compared with the overall performance of Mahu oilfield, the drainage efficiency and estimated recovery factor of this pilot were significantly boosted with improved economics.
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Based on the abundant core data of oil sands in the Mackay river in Canada, the termination frequency of muddy interlayers was counted to predict the extension range of interlayers using a queuing theory model, and then the quantitative relationship between the thickness and extension length of muddy interlayer was established. An equivalent upscaling method of geologic model based on tortuous paths under the effects of muddy interlayer has been proposed. Single muddy interlayers in each coarse grid are tracked and identified, and the average length, width and proportion of muddy interlayer in each coarse grid are determined by using the geological connectivity tracing algorithm. The average fluid flow length of tortuous path under the influence of muddy interlayer is calculated. Based on the Darcy formula, the formula calculating average permeability in the coarsened grid is deduced to work out the permeability of equivalent coarsened grid. The comparison of coarsening results of the oil sand reservoir of Mackay River with actual development indexes shows that the equivalent upscaling method of muddy interlayer by tortuous path calculation can reflect the blocking effect of muddy interlayer very well, and better reflect the effects of geological condition on production.
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Experiments of electrical responses of waterflooded layers were carried out on porous, fractured, porous-fractured and composite cores taken from carbonate reservoirs in the Zananor Oilfield, Kazakhstan to find out the effects of injected water salinity on electrical responses of carbonate reservoirs. On the basis of the experimental results and the mathematical model of calculating oil-water relative permeability of porous reservoirs by resistivity and the relative permeability model of two-phase flow in fractured reservoirs, the classification standards of water-flooded layers suitable for carbonate reservoirs with complex pore structure were established. The results show that the salinity of injected water is the main factor affecting the resistivity of carbonate reservoir. When low salinity water (fresh water) is injected, the relationship curve between resistivity and water saturation is U-shaped. When high salinity water (salt water) is injected, the curve is L-shaped. The classification criteria of water-flooded layers for carbonate reservoirs are as follows: (1) In porous reservoirs, the water cut (fw) is less than or equal to 5% in oil layers, 5%-20% in weak water-flooded layers, 20%-50% in moderately water-flooded layers, and greater than 50% in strong water-flooded layers. (2) For fractured, porous-fractured and composite reservoirs, the oil layers, weakly water-flooded layers, moderately water-flooded layers, and severely water-flooded layers have a water content of less than or equal to 5%, 5% and 10%, 10% to 50%, and larger than 50% respectively.
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By comparing numerical simulation results of single-porosity and dual-porosity models, the significant effect of reinfiltration to naturally fractured reservoirs was confirmed. A new governing equation was proposed for oil drainage in a matrix block under the reinfiltration process. Utilizing inspectional analysis, a dimensionless equation suitable for scaling of recovery curves for matrix blocks under reinfiltration has been obtained. By the design of experiments, test cases with different rock and fluid properties were defined to confirm the scope of the presented equation. The defined cases were simulated using a realistic numerical simulation approach. This method can estimate the oil amount getting into the matrix block through reinfiltration, help simulate the oil drainage process in naturally fractured reservoirs accurately, and predict the recovery rate of matrix block in the early to middle periods of production. Using the defined scaling equation in the dual-porosity model can improve the accuracy of the predicted recovery rate.
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Due to engineering technology and development costs, a considerable amount of proven oil and gas resources in China are difficult to develop, becoming reserves difficult to produce. Based on the successful development of some domestic oil and gas reserves difficult to recover, this article summarizes the "four in one" petroleum engineering synergetic management model to promote the effective development of these reserves. This model draws on the essence of management theories, such as system theory, cybernetics and synergetic theory, and proposes the development idea of value creating and sharing for this type of reserves. By adopting the new management method of mechanism synergy, speciality synergy, process synergy and industrial chain synergy, this model effectively overcomes the decentralization of management responsibility, different management objectives, great risks of engineering and technological innovation, and the large number of uncertain factors in project construction, and can stimulate the vitality and power of active coordination of project participants, to effectively realize the synergetic innovation of engineering technology and synergetic cost reduction of the whole chain, reduction of the balanced oil price of the project, and dispersion of the project investment risk. By adopting this model, a large proportion of difficult-to-produce reserves have been liberated, realizing the effective utilization of the difficult-to-produce oil and gas resources, and making the reserves an important supplement to ensure national energy security.
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The predictions by drilling-related mechanical and geological models are in some degree inaccurate due to non-unique solution of seismic velocity model. To address this problem, a new drilling technology guided by well-seismic information integration is proposed which consists of seismic velocity update of drilled formations, seismic velocity prediction of the formation ahead of drilling bit, and the prediction of geological feature and drilling geological environmental factors ahead of bit. In this technology, real information (velocity, formation and depth) behind the drilling bit and local pre-stack seismic data around the wellbore being drilled are used to correct the primitive seismic velocity field for a re-migration of seismic data and to update geological features and drilling geological environmental factors ahead of the drilling bit. Field application shows that this technology can describe and predict the geological features, drilling geological environmental factors and complex drilling problems ahead of the bit timely and improve the prediction efficiency and accuracy greatly. These new updated results are able to provide scientific basis for optimizing drilling decisions.
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To formulate fluids with flowback water, produced water directly to improve the utilization rate of recycling and reduce the adsorption damage of slick water to reservoirs, a high salt tolerance and low adsorption drag reducer was designed and prepared by introducing polar cation fragments to enhance the non-covalent interactions between the chains. The drag reducer was characterized by IR and NMR. Friction resistance and viscosity tests were conducted to evaluate its salt resistance property. Static adsorption and dynamic adsorption retention tests were carried out to evaluate the damage of this reducer to shale reservoirs. The introduction of cation units into the molecular structure can weak the shielding effect of metal cations to some extent, so the drag reducer can keep a stable molecular structure and good resistant reducing performance under high salinity. The enhancement of non-covalent interaction between chains decreased the free polarity sites, further reduced the possibility of hydrogen bonding between drag reducer molecules and shale. In high salinity condition, both the adsorption capacity of the drag reducer on the shale surface and the average damage rate to the core permeability are low. Compared with the conventional salt-tolerant system, the overall liquid cost was reduced by 17% and the production per well increased by 44%. The application of this slick water system has achieved remarkable results.
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The Chang 73 sub-member of Triassic Yanchang Formation in the Ordos Basin was taken as an example and the lamina types and combinations, reservoir space features and shale oil enrichment patterns in organic-rich shale strata were investigated using core observation, thin section analysis, XRF element measurement, XRD analysis, SEM, high solution laser Raman spectroscopy analysis, and micro-FTIR spectroscopy analysis, etc. According to the mineral composition and thickness of the laminae, the Chang 73 organic-rich shales have four major types of laminae, tuff-rich lamina, organic-rich lamina, silt-grade feldspar-quartz lamina and clay lamina. They have two kinds of shale oil-bearing layers, “organic-rich lamina + silt-grade feldspar-quartz lamina” and “organic-rich lamina + tuff-rich lamina” layers. In the “organic-rich + silt-grade feldspar-quartz” laminae combination shale strata, oil was characterized by relative high maturation, and always filled in K-feldspar dissolution pores in the silt-grade feldspar-quartz laminae, forming oil generation, migration and accumulation process between laminae inside the organic shales. In the “organic-rich + tuff-rich lamina” binary laminae combination shale strata, however, the reservoir properties were poor in organic-rich shales, the oil maturation was relatively lower, and mainly accumulated in the intergranular pores of interbedded thin-layered sandstones. The oil generation, migration and accumulation mainly occurred between organic-rich shales and interbedded thin-layered sandstones.
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Based on the research of the formation mechanism and evolution rule of hydrate flow obstacle during deep-water gas well testing, a new method for the prevention of hydrate flow obstacle based on safety testing window is proposed by changing the previous idea of "preventing formation" to the idea of "allowing formation, preventing plugging". The results show that the effective inner diameter of the testing tubing and the wellhead pressure decrease gradually with the formation and precipitation of hydrates during deep-water gas well testing, and it presents three typical processes of slow, fast and sudden changes. There is a safety testing window during deep-water gas well testing. The safety testing window of deep-water gas well testing decreases first and then increases with the increase of gas production rate, and increases with the increase of hydrate inhibitor concentrations. In the case with different testing production rates, a reasonable testing order with alternate low and high gas production rates has been proposed to further reduce the dosage of hydrate inhibitor and even avoid the use of hydrate inhibitors considering the decomposition and fall-off of hydrates. Compared with the traditional methods, the new method based on safety testing window can reduce the dosage of hydrate inhibitor by more than 50%.
