The shale strata of continental lacustrine basins in China are widely distributed in multiple layers and have great potentials for oil resources. Vigorously increasing the intensity of intra-source oil exploration and development is of great significance for ensuring national oil security^{[1,2,3,4,5,6,7,8]}. In the past two years, China National Petroleum Corporation (CNPC) has continued to increase its efforts in basic scientific research and field experiments, and has made important progresses in the understandings of petroleum geology in key strata, and has made important breakthroughs in the pilot tests of petroleum exploration and development in key exploration areas, showing good development prospects^{[1-3, 5]}. Efforts are being made to build three systems of theory and technology, demonstration projects, and standards for continental intra-source petroleum and to promote continental intra-source unconventional petroleum to achieve revolutionary breakthroughs as soon as possible and achieve scale development.

Based on the latest practice of unconventional oil exploration and development in China’s continental sources, the CNPC research team elaborates on the geological connotation, primary types and geological characteristics of oil accumulation and “sweet spots” of continental hydrocarbon accumulation inside source kitchen, summarizes the major theoretical understandings and key technologies for the exploration and development of intra-source petroleum, prospects the development potential and status of intra-source petroleum resources, and analyzes the major theoretical and technical challenges and major countermeasures, in the hope of providing references for promoting intensive research on petroleum geological evaluation and accelerating exploration and development of continental intra-source petroleum.

After nearly 60 years of technological research and production, the American shale oil and gas has achieved major innovations in all aspects of cognition, technology, management and strategy, thereby setting off the “shale revolution”^[9]. The American shale oil and gas reserves and production have surged that the oil production of shale formations in 2019 reached 3.86×10⁸ t, accounting for 52% of total oil production; shale gas production in 2019 reaching 7174×10⁸ m³ which accounts for 78% of the total gas production. The United States has achieved effective replacement of oil and natural gas resources that it has become the world’s largest oil and natural gas producer, nearly achieving the independent self-sufficiency of energy supply^[10,11]. The revolutionary development model of oil and gas in unconventional shale formations in the United States has brought China the most likely trajectory of oil and gas development in shale formations in the future. China’s marine shale gas has achieved large-scale commercial exploitation, and China’s continental shale oil has also made great major breakthroughs and progresses.

In recent years, CNPC has made substantial and important discoveries and breakthroughs in petroleum geology research, risk exploration, overall exploration, development testing, and productivity construction efforts in continental intra-source petroleum^[1,2]. Large-scale production in multiple exploration areas in the Ordos, Junggar, Songliao, and Bohai Bay basins have been realized^{[1-2, 12-24]}. A series of key technologies such as volumetric fracturing of horizontal wells have been developed. The resources and technical foundations for accelerating the large-scale development of unconventional oil within continental sources gradually are consolidated successively (Fig. 1).

In the Ordos Basin, the evaluation of “sweet spots (sections)” and the exploration of production increase by horizontal well volumetric fracturing and platform industrial deployment was conducted. The discovery of 10×10⁸ t was achieved in the Upper Triassic Chang 7₁ and Chang 7₂ oil layers in the Qingcheng large oilfield. The risk exploration well of Chengye horizontal well successfully broke through the oil production barrier at Chang 7₃ Member. Horizontal well research tests were carried out in the well areas of Xi233, Zhuang183, and Ning89 etc., which initially formed the effective implementation of development of volume fracturing in long horizontal wells for supply energy in the Chang 7 Member.

The Permian Lucaogou Formation in the Jimusaer Sag of the Junggar Basin has continued to carry out comparative tests of development methods to improve the production of inter-well and longitudinal oil layers, and has achieved large-scale reserves and production. In the Well Maye 1, the Permian Fengcheng Formation obtained 50.6 m³ of industrial oil flow per day in the 4579-4852 m interval by multi-layer fracturing in vertical wells.

The shale oil exploration and development in the Cretaceous Qingshankou Formation of the Songliao Basin has made major breakthroughs and progress. The Qing 1 Member in the Gulong Sag in the northern part of the basin obtained high-yield industrial oil flow of more than 35 m³ at 2742- 4214 m depth in the Wells Guyeyouping 1 and Yingye 1. Vertical well fracture network fracturing, and horizontal well volume fracturing development tests have achieved good results. The Qian’an-Da’an shale oil in the Changling Sag in the southern part of the basin also shows the potential for profitable production.

Important progresses have been made in the exploration and development of multi-layered shale oil in multiple hydrocarbon-rich sags in the Bohai Bay Basin. Two horizontal wells in Guandong area of the Cangdong Sag were tested in the Paleogene Kong 2 Member and achieved a daily output of more than 60 tons. The first well group of C1 sweet spot achieved stable production, and the shale oil of the Kong 2 Member achieved breakthroughs in single well production and well group production construction. The shale oil in the Sha 3 Member in the Qikou Sag was newly discovered, and 4 wells obtained industrial oil flow. New discoveries and new progresses have been made in shale oil exploration in the Sha 4 Member in the Leijia area in the West Sag, the Sha 4 Member in the Damintun Sag, the Sha 3 Member in the Shulu Sag, the Sha 1 Member in the Raoyang Sag, and the Sha 1 Member in the Nanpu Sag.

Since the discovery of oil-bearing tuff reservoirs in the Permian Tiaohu Formation in the Malang Sag of the Santanghu Basin, the targeted technical research has been carried out on the integrated evaluation and production construction, and the 3698×10⁴ t large-scale reserves have been confirmed and profitable production has been achieved.

Progresses have also been made in unconventional oil exploration in the Jurassic Da’anzhai Member of the central- northern Sichuan Basin, Paleogene E₃² in the Yingxi area and Neogene N₁ in the Fengxi area of the western Qaidam Basin.

The continental organic-rich shale strata are generally continuously distributed in a large area in the middle of a lacustrine basin, and they are generally oil-bearing as a whole. A large number of organic-rich shale strata drilled exhibit abundant oil and gas shows, proving the existence of large-scale petroleum resources in the shale source strata^{[25,26,27,28,29]}. Continental hydrocarbon accumulation inside source kitchen refers to the accumulation of liquid-rich hydrocarbons that occurs in continental organic-rich shale source rocks, in situ retained or captured within the source rocks. The intra-source petroleum mainly exists in two occurrence states (adsorbed state and free state) which are mainly stored in micro- and nano-scale pore throats and fracture systems. The free hydrocarbons are generally distributed along the foliage, laminae or micro-fractures parallel to it. Petroleum accumulation within the source has no natural production capacity or is lower than the lower limit of commercial oil production, and special technological measures are required to obtain commercial oil production. There is a clear difference between hydrocarbon accumulation inside source kitchen and off-source conventional petroleum reservoirs. Hydrocarbon accumulation inside source kitchen is characterized by relatively fine lithology but generally the containment of organic matter, generally tighter reservoirs but overall oil bearing, diverse source-reservoir combinations but a high probability of retention or capture of hydrocarbons, worse fluid differentiation but generally relatively large formation pressure, and lower resources abundance but larger resources scale. However, the off-source conventional petroleum reservoirs have coarser lithology but generally do not contain organic matter; the reservoir physical properties are generally better but not always contain oil; the trap types are diverse but the probability of capturing hydrocarbons is relatively low; fluid differentiation is better but formation pressure is variable; the resources abundance is higher, but the resources scale is relatively small. These differences determine that the exploration and development of hydrocarbon accumulation inside source kitchen has lower resources risks and higher technical and management requirements than off-source conventional petroleum reservoirs.

Evaluating and transforming the favorable reservoir “sweet spots” of oil-enriched intra-source shale strata is the major task of exploration and development of continental intra- source oil accumulation^[28,29,30]. The “sweet spot” of continental intra-source oil accumulation refers to the distribution area (section) of favorable reservoir with richer oil, better physical properties, easier transformation, and commercial development value under existing economic and technological conditions within the continental shale source strata and under the overall oil-bearing background. Important parameters for “sweet spot” evaluation include porosity, oil saturation, oil content, maturity, pressure coefficient, gas-oil ratio, lamella and micro-fracture development, organic matter abundance, brittle mineral content, horizontal stress difference, and the likes. Among them, free hydrocarbon, texture and natural fracture density, pressure coefficient, gas-oil ratio, compressibility, and the likes are the key indicators that affect development.

The hydrocarbon accumulation inside source kitchen in China is substantially occurred in the shale strata of continental lacustrine basins. Their rock types and mineral compositions are complex, forming a unique and diverse type of lacustrine shale reservoir “sweet spot”, which is a favorable place for intra-source capture and in-situ retention within the source rock. According to the geological conditions and sedimentary characteristics, the “sweet spots” of China’s continental shale reservoirs can be roughly divided into three types: interlayer type, hybrid sediment type and shale type (Fig. 2). (1) The “sweet spot” of interlayer type can be intercalated with sandstone, limestone, tuff or other lithologies. The “sweet spot” of sandstone type is the most important type, showing the characteristics of multi-layers, multi-types, and large-area distribution. The oil is mainly enriched in the form of thin-interbedded “sweet spots” within the source, for example, the “sweet spot” of sandstone type in the Chang 7 Member of the central lacustrine basin in the Ordos Basin, the “sweet spot” of tuff type in the Tiaohu Formation in the Santanghu Basin, and the likes^{[14, 21]}. (2) The “sweet spot” of hybrid type is mainly the laminated hybrid shale strata formed by various factors, such as changes in climate rhythm and hydrodynamic conditions, mixed accumulation of different provenances and organic matter flocculation, etc. Such “sweet spots” include the sandy dolomite “sweet spot” of the Lucaogou Formation in the Jimusaer Sag of the Junggar Basin, the dolomitic “sweet spot” in the Kong 2 Member of the Cangdong Sag in the Bohai Bay Basin, limy “sweet spot” of the Jurassic Da’anzhai Member in the Sichuan Basin^{[19-20, 23-24]}. (3) The “sweet spot” of shale type is mainly pure shale with effective pore space and certain seepage capacity, which are both oil-generating and oil-bearing layers, such as laminar and foliage shale of the Cretaceous Qingshankou Formation in the Songliao Basin^[31,32].

The deposition of continental lacustrine basins in China is evidently controlled by factors such as regional structure, climate, hydrodynamic conditions, water medium properties, and biological activities, with structure and climate being the major controlling factors. The regional structure, which is long-scale, controls the large pattern of lacustrine basin spatial distribution and regional macroscopic characteristics. The climate controls precipitation and temperature, and has a more direct impact on the shaping and impact of lacustrine basins, mostly in short-to-medium scales^[33,34]. Lacustrine basins can be divided into freshwater lacustrine basins and saltwater lacustrine basins. Freshwater lacustrine basins are mostly developed in humid climate regions. The water level varies with the seasons, and most of them are open-flow lacustrine basins. The sediments are mainly from rivers and are dominated by mechanical sedimentation. The transported matters are deposited in order of particle size along the transport direction and are mainly dominated by clastic sediments, represented by the lacustrine basin during the deposition of the Yanchang Formation in the Ordos Basin and the lacustrine basin during the deposition of the Qingshankou Formation in the Songliao Basin. Saltwater lacustrine basins are developed in arid climate zones and are generally closed lacustrine basins, dominated by chemical sedimentation. The solution precipitates in the order of mineral solubility, concentration, and the likes, the rocks are mainly carbonate rock, salt rock, gypsum salt rock and other deposits, represented by the Bohai Bay lacustrine basin and Qaidam lacustrine basin during the Cenozoic deposition period. In addition, there are transitional types of lacustrine basin sedimentation. Both mechanical sedimentation and chemical sedimentation have important influences, forming a mixture of clastic rock and carbonate rock. This is represented by the lacustrine basin in the depositional period of the Lucaogou Formation in the Jimusaer Sag in the eastern Junggar Basin. There are also special types of lacustrine basin sedimentation. For example, lacustrine basins strongly affected by volcanic activity formed volcanic clastic rock deposits and tuff-rich deposits can be represented by the lacustrine basin during the deposition of Tiaohu Formation in Malang Sag, Santanghu Basin.

Continental lacustrine basin deposits have clastic input deposits and endogenous carbonate deposits, which have an important controlling effect on the formation of reservoir “sweet spots” of organic-rich hydrocarbon source strata and intra-source oil accumulation. It is rich in felsic and dolomite brittle minerals, lamella and micro-fractures are generally the “sweet spot” of the reservoir. (1) Terrigenous debris is transported from the lacustrine bank to lacustrine center, from shallow water area to deep water area, the hydrodynamic strength gradually weakens, and the mechanical sedimentation differentiation forms a lithological and lithofacies zonation with sediment changing from coarse to fine grain size. The sedimentary environment of continental organic-rich shale is a long-term sinking lacustrine basin covered by deeper water bodies. The weakly reducing and reducing environment in the semi-deep to deep lacustrine area is conducive to the rapid burial of organic matter and the formation of continental oil reservoirs, as well as the formation of oil accumulation within the source. The semi-deep lacustrine is located in the deep-shallow lacustrine transition zone below the wave base, which is a weakly reducing-reducing environment. It is dominated by mud shale, mostly organic-rich mud shale or silty-carbonate mud shale. Many types of laminae are generally developed, often with thin interbedding or lenses of siltstone and carbonate rocks. The deep lacustrine is located in a quiet water area below the wave base. The deepest part of the water in the lacustrine basin and the hypoxic reduction environment are the most favorable oil-generating facies. The sedimentary lithology is fine in grain size, deep in color, high in organic matter content, and stable in lateral distribution, with the development of high-density foliage^[35]. (2) Endogenous carbonate rocks are mainly present in saltwater enclosed lacustrine basins, offshore salty and alkalinized lacustrine basins, lagoon basins and salt lacustrine^[36,37,38]. The interval in which the lacustrine carbonate rock is mostly developed is gentle in tectonic activity generally and slow compensation for the subsidence and sedimentation of lacustrine basin. The lacustrine basin is open and the water area is wide. Algae and other organisms grow and propagate in large quantities. On plane, carbonate rocks in different facies zones are distributed in a continuous or discontinuous belt around the lacustrine shore. The top of the relatively uplifted positive topography or slope zone in the shallow lacustrine area develops shoal facies and reef facies limestone, and semi-deep lacustrine-deep lacustrine areas can form micrite and marl. Most of them are sandwiched in black shale in thin layers. The climate and depositional conditions change frequently and periodically, which leads to the characteristics of a large number of layers, thin single layers, and rhythmic changes in the formations of carbonate rocks^[36].

Affected by many factors such as climate, transgression, provenance, volcanism, lacustrine bottom hydrothermal fluid, China’s continental freshwater and saltwater lacustrine basins have formed complex rock types and sedimentary structures. (1) The Early Cretaceous Qingshankou Formation in the Songliao Basin was a warm and humid freshwater-slightly salty lacustrine basin, with periodic dry and wet changes in each section in the longitudinal direction^[34,35]. When the climate is warm and humid, and the lacustrine basin is open, the rainfall is great. The surface runoff can bring a large amount of terrigenous clastic material and calcium and magnesium ion components to the lacustrine basin, and it is easy to form clastic rocks or hybrid rocks. In the relatively dry period, the river is uplifted by lacustrine water, the input of terrigenous debris into the lacustrine is rapidly reduced, and the water is clear and suitable for biological reproduction, resulting in the better development of carbonate rocks and biological carbonate rocks. The large-scale lacustrine transgression in the early stage of Qingshankou Formation formed a weak, anoxic, and strongly reducing sedimentary environment from continental sources, which was conducive to the generation, enrichment and later preservation of organic matter, and also promoted the formation of foliage, silty, dolomitic and limy laminas in shale reservoirs. (2) The Paleogene in the Bohai Bay Basin was a closed saltwater lacustrine basin. The lakes in sags were separated from each other and had poor connectivity. The organic-rich shale deposition period was a deep-water, brackish to semi-saline lacustrine. The internal density, temperature, and salinity differences of the lacustrine caused long-term stratification, and the fine-grained deposition was seasonal, therefore it is easy to form sedimentary structures of lamina and foliage^{[33, 37-38]}.

The shale strata of China’s continental lacustrine basins are generally rich in texture, and the thicknesses of the shale layers are generally on the order of micrometers to millimeters, and they are distributed continuously or intermittently in the horizontal direction. Changes in climate and sedimentary environment during the sedimentary period of the lacustrine basin affect water temperature, salinity, terrigenous clastic content, and internal clastic carbonate content. Changes in provenance supply sediments affect sediment composition, grain size, organic matter content, and the likes to form different layers. Generally, they show a binary or ternary structure, with the former being a high-frequency interlayer of silt and clay/organic matter or carbonate and clay/organic matter, and the latter being a high-frequency interlayer of silt, clay/organic matter and carbonate matter. The continental shale strata have fine grain size, small individual pores, ranging from a few nanometers to tens of nanometers, but the total pores are large, and the tiny pores provide accumulation place for the occurrence of oil within the source.

At present, meticulous research, innovative methods and technologies were used to pinpoint the “sweet spot intervals”, to precisely select the “sweet spot zones”, to drill into the “sweet spot layers” and fracture the “sweet spot volumes”, and benefit from favorable geological conditions, higher formation pressure and applicable engineering technology. The medium-high mature shale strata intercalated by thin sand layer of the Chang 7 Member in the central lacustrine basin in the Ordos Basin (Fig. 3), and the hybrid-type shale strata in the moderate mature area of the Lucaogou Formation in the Jimusaer Sag in the eastern Junggar Basin (Fig. 4), laminar and high-density shale in the Qing 1 Member in the Gulong Sag of the Songliao Basin (Fig. 5), dolomitic shale strata in the Cangdong Sag of the Bohai Bay Basin have obtained industrial oil and gas from intra-source oil wells. The exploration and development of intra-source oil accumulation have achieved breakthroughs or showed good development prospects.

After recent years of scientific and technological researches, we have gained important theoretical cognitions of continental intra-source oil accumulation in basic research on source-reservoir quality, resource evaluation, sweet spot parameters, and development geology^{[1, 3-5, 8, 39-40]}.

(1) Two types of high-quality shale in salt water and freshwater lacustrine basins are continuously distributed in large areas and can generate hydrocarbons on a large scale. Since the Permian, China has extensively developed large-scale freshwater and saline water depressions and faulted lacustrine basins with long-term continuous subsidence, which can preserve organic matter and form high-quality source rocks. The rocks such as shale, mudstone, siltstone, limestone, dolomite, tuffaceous rock can contain organic matter, mainly Type I-II organic matter. The thermal evolution degree is mostly in the stage of liquid oil generation. The major source rocks in mature areas have great resource potential and are rich in a large number of mobile resources. The semi-deep to deep water lacustrine facies is the most favorable facies belt for the formation of source rocks with high abundance of sedimentary organic matter that it can form favorable areas (sections) of high abundance shale. Organic laminae are widely present in organic-rich shale, and organic matter is amorphous and irregular particle of clot or floc, which are in the form of laminae and bands.

(2) The shale strata have developed diverse favorable reservoirs (felsic, dolomite, and shale types) with large-scale storage capacity. The sedimentary types of shale strata are diverse, mainly lacustrine fine-grained mixed-source sediments. Affected by many factors, such as changes in climate rhythm and hydrodynamic conditions, mixed provenance, organic matter flocculation, shale strata are widely developed with interlayers, massive, laminar and foliage structures. The thickness of a single layer of the laminar structure is generally on the order of micrometers to millimeters, and the thickness of a single layer of lamella structure is generally on the order of millimeters to micrometers. The composition of adjacent layers is quite different, and the lithological interface shows abrupt changes. At the same time, the inter-granular pores, inter-crystal pores, and foliage fractures of the micro- to nano-scale pore throat system are also widely developed. The inter-granular pores are mainly formed by minerals such as felsic, dolomite and clay minerals. The inter-crystal pores are mainly void spaces from the dissolution of secondary minerals such as quartz, feldspar, calcite. The shale fracture is also an important type of storage space. For example, the density of the shale fracture in the Qing 1 Member can reach 1000-3000 bars/m^[32], providing a good accumulation space for intra-source oil enrichment.

(3) The continental shale strata have a high degree of oil charging, a large number of oil-bearing strata, a wide distribution area, a large scale of oil resources, and there are multiple types of local favorable “sweet spots” of reservoir. Exploration research and preliminary practice in key exploration areas in recent years have confirmed that China’s continental shale strata are characterized by large-scale intra-source oil accumulation, multi-layered systems, and large-scale distribution. There are many types of intra-source oil accumulation, and the lithology combinations are mainly a large set of thick source rocks and siltstone, carbonate rock, shale and other interlayers or lamellae with better development of pores in the source rock. They are characterized by coexistence of source rock and reservoir, hydrocarbon charging by pressurization, and large-scale accumulation. The accumulation is not controlled by structure, and local reservoirs with high abundance and high oil saturation can form “sweet spots” with high production potential.

(4) The continental shale strata are characterized by overpressures, high gas-to-oil ratios, natural fractures and brittle interbedded layers, with great development potential. Shale formations in some areas and strata have better oiliness and compressibility, which are conducive to the stable production and effective development of oil within the source rocks, and are major targets of recent exploration and development. These shale formations have good hydrocarbon-generating parent materials, high organic carbon content, higher thermal evolution degree, larger free hydrocarbon content, higher oil saturation, and higher gas-to-oil ratio in some areas, better crude oil quality, and strong mobility, with good oil and gas-bearing properties. In the shale strata, the dense sections of natural fractures and lamina, brittle interlayers such as silty, calcareous, and tuffaceous interlayers or sections with laminar development, have better compressibility. In addition, in areas with well-developed shale intervals and high thermal evolution degree, clay minerals are transformed into illite, which also has good brittleness and compressibility.

The practice of exploration and development of continental intra-source oil is promoting the formation of a series of key exploration and development technologies centered on “sweet spot exploration” and “volume development”^{[1-2, 41-43]}. The promotion and application have preliminarily achieved evident application results (Table 1).

With “sweet spot” as the research object, the technologies of experimental analysis, basic geology, well logging and seismic data for multi-scale fine characterization, evaluation and prediction were innovatively conducted in the intra-source exploration. The “sweet spot exploration” technology series are being established and improved. Through a set of research and development of key parameter experimental methods and technologies, one-time acquisition of accurate coring wells, well logging and mud logging geological data, integrated acquisition and processing of new and old 3D seismic data, the depiction methods for favorable reservoir “sweet spot interval” and “sweet spot zone” have been established, and the technologies for fine evaluation and seismic prediction of “sweet spots” on plane and in vertical direction by integration of geology and engineering have been innovated. We carried out exploration, evaluation and deployment of “sweet spots” in different regions, different horizons, and different maturity levels, and strengthened overall research and technical tackling of key continental shale strata and zones, which have effectively guided well location deployment, horizontal well steering, and optimized design of fracturing schemes. A set of intra-source oil accumulation “reservoir sweet spots” exploration mode has been preliminarily formed, and further improvements can be better applied in controlling the spatial distribution and resource scale of intra-source oil accumulation as a whole in each exploration area. (1) For the exploration area in the Ordos Basin, it is focused on intensive evaluation of favorable areas in the central lacustrine basin, optimizing exploration enrichment areas and development and production areas, increasing 3D seismic deployment, optimizing deployment of exploration wells, and strengthening the evaluation and prediction of intra-source “reservoir sweet spots” in Chang 7 Member sandstone and cognitions the overall scale of reserves and the overall development of the core area as soon as possible. (2) In the Junggar exploration area, it should continue to strengthen the fine prediction and development of the two sets of mixed-type “reservoir sweet spots” of the upper and lower Lucaogou Formation in the Jimusaer Sag, and steadily advance the construction of a million-ton national demonstration zone and promote the overall evaluation and overall breakthrough of the dolomitic “reservoir sweet spot” of the Fengcheng Formation in the Mahu Sag. (3) The Songliao exploration area will focus on the favorable zone of light oil in the northern part of the basin, taking into account the preliminary exploration and evaluation of intra-source oil in the southern part of the basin, and further strengthen the fine geology evaluation of the “sweet spots” of the lower laminar and foliage reservoirs of the Qingshankou Formation, control the “sweet spot zones” and implement the scale of resources as soon as possible. (4) In the Bohai Bay exploration area, it will focus on the “sweet spot intervals” of different strata, such as the dolomite type in the Kong 2 Member in the Cangdong Sag and the overall scale of resources will be determined. (5) For shale strata such as the Lucaogou Formation in the Santanghu exploration area and the Jurassic Da’anzhai Member in the Sichuan exploration area, the identification and prediction of different types of “reservoir sweet spots” will be carried out in a targeted manner, and preparatory exploration and evaluation will be expanded, so as to make a breakthrough as soon as possible.

Taking the “sweet spots” as the object of operation, innovatively establish the “volume development” technology system of development methods, well layout methods, fracturing mechanism, oil displacement mechanism, development and production allocation, enhanced oil recovery, economic model, and post-production for intra-source oil accumulation. The researches have formed a series of “volume development” engineering technologies^[41], which provides basic technical support for the overall development of intra-source oil (Table 1). (1) The technology of long horizontal wells, small well spacing, and large cluster wells breaks the traditional water injection method of conventional oil reservoirs. In the initial stage, the development method of imbibition replacement is adopted to supplement energy in advance. The length of the horizontal section is optimized, and the large cluster of three-dimensional well layout can fully increase utilization degree of reserves. The optimization of well spacing can improve oil production rate and recovery ratio. (2) Cluster-type 3D horizontal well drilling and completion technology with excellent and fast drilling. Through the integration of excellent and fast drilling, strengthening small-site and large-well cluster drilling and improving friction and drag reduction technologies, the drilling speed has be improved, the number of completed wells on a single platform, and the horizontal section have been improved. It is necessary to continue to strengthen the technical research of horizontal well drilling and completion. In the design, firstly realize two drilling and two trips, further shortening the drilling cycle and reducing costs. (3) Subdividing volume fracturing technology to increase single well production. After long-term exploration, many indoor studies and field practices, a key technology of horizontal well volume fracturing has been formed, which can effectively improve the degree of fracture control and single well production. The next step is to strengthen studies in the following three aspects. First, it is necessary to strengthen studies in the high-efficiency staged fracturing technology test of horizontal wells, to further optimize the fracturing fluid formula and proppant, to optimize the number of fracturing sections and clusters, to optimize the fracturing scale, and to form a suitable supporting fracturing technology system, thereby to improve the efficiency of fracturing construction. Secondly, it is necessary to strengthen the oil test and drainage test with different pressure differences, which can not only ensure that no sand is produced, but also maintains a stable pressure and obtain stable production. Thirdly, it is necessary to strengthen researches on the fracture monitoring in horizontal well and liquid production monitoring technology. (4) To take the platform-based and factory-based operation technology as the core, through platform-based well deployment, batch drilling, zipper fracturing, digital and intelligent supporting oil production technology, effectively improve efficiency, reduce ground investment, build full life cycle management system with project management, market operation, platform-based well deployment, factory-based operations, and intelligent management and control. (5) To research and develop key technologies and equipment in advance, such as magnetic steering drilling and in-situ conversion (heating system etc.) to provide reserve technology for large-scale developing moderate-low mature shale oil.

The pilot test is an important tool for the practice of “sweet spot exploration” and “volume development”, and provides an important scientific basis for the large-scale, economic and effective development of intra-source oil. For different types of “reservoir sweet spots”, we selected pilot test areas, carried out pilot development tests of platform-based horizontal wells, focused on productivity, economics and process evaluations, took all basic data, gradually increased the scale of tests. Horizontal well layout technology for horizontal sections, small well spacing, and large well clusters has been popularized and applied. The Changqing Oilfield is the pioneer and demonstrator of China’s intra-source unconventional oil pilot test. Since 2013, it has explored ways to effectively increase single-well production by intra-source oil accumulation in the Chang 7 Member. Through systematic research and test of stimulation mechanism, fracture design, high-efficiency and low-cost fracturing process and tool materials, innovative subdivision cutting, energy supplement, imbibition flooding and fracturing design, overall fracturing optimization of the research platform, soluble ball seat fracturing, and key technologies of oil flooding fracturing fluid, implementation of economic optimization of key parameters, cost reduction measures such as fine control of multiple clusters of fractures, self-developed and self-produced tool materials, and exploration of “multi-layer system, three-dimensional, large well clusters, and industrialization” have successively built intra-source oil enrichment and development pilot areas such as Xi 233, Ningping 1, Zhuang 183, and Ning 89. 25 horizontal wells obtain an average daily output of over 100 m³ in oil testing, integrated innovation with 5 major technology series and 18 supporting technologies centered on long horizontal wells, small well spacing, large well clusters, and subdivided cutting volume fracturing. The large-cluster horizontal well group with the largest number of wells and the longest section in China has created a new record of fracturing construction with the largest number of single well intervals and the highest construction efficiency in a single day, achieving remarkable results in large-scale and efficient development. The Chang 7 Member in the central lacustrine basin of the Ordos Basin, the “lower sweet spot interval” of the Lucaogou Formation in the Jimusaer Sag of the Junggar Basin, the Qing 1 Member of the Songliao Basin, and the Kong 2 Member of the Cangdong Sag in the Bohai Bay Basin are major strata for increasing reserves and production of unconventional oil of CNPC in the “14th Five-Year Plan”. It still needs to focus on the bottleneck problem and carry out targeted pilot tests: (1) The Ordos exploration area needs to focus on the large-scale production of new formations and blocks in the Chang 7 Member and the formation energy supplement technology. Adhere to the principle of pilot test first, to carry out development field tests, and focus on solving the issues such as the pressure channeling, inter-well interference, full reservoir reformation, determination of major production layers, liquid production profile measurement, segmented inter-well tracer, downhole micro-seismic operation, utilization degree of horizontal interval, and the development of double-layer horizontal wells, thereby providing a solid basis for the large-scale and effective development of intra-source oil in the Chang 7 Member. (2) The Junggar exploration area needs to focus on strengthening the development pilot test of the horizontal well group for the “lower sweet spot interval” in the Lucaogou Formation in the Jimusaer Sag, and optimize the new model of “small well spacing, three-dimensional interlocking of large well clusters, industrialization” for productivity construction in the well area, and test the prime development effect of injecting CO₂ at the same time. (3) The Songliao exploration area needs to focus on the development test of the horizontal well pilot test area in the Gulong Sag, highlighting primary supporting technology test of different aspects such as length, well spacing, well depth, trajectory. (4) The Bohai Bay exploration area focuses on the construction of the Kong 2 Member in the Guanxi area of the Cangdong Sag, and to build a beneficial production test area by combining horizontal wells with highly deviated wells.

The potentials of medium-high mature oil resources in thecontinental sources of China are great, mainly in the Ordos, Songliao, Junggar and Bohai Bay basins, etc. They are the realistic replacement of oil resources in the future, and have a promising prospect for exploration and development. The Ordos Basin, Songliao Basin, and Bohai Bay Basin are three super basins of intra-source petroleum with a resource scale of 50×10⁸ t or more, and the Junggar Basin and Sichuan Basin are two large basins of intra-source petroleum with a resources scale of 20×10⁸ t or more, further the Qaidam Basin and Santanghu Basin are also important basins of intra-source oil on a scale of 100 million tons (Fig. 1). The Chang 7 Member of the Ordos Basin, the Lucaogou Formation and Fengcheng Formation of Junggar Basin, the Kong 2 Member, Sha 4, Sha 3 and Sha 1 Members of the Bohai Bay Basin, and the Qingshankou Formation of the Songliao Basin are important areas for future intra-source oil exploration. The Da’anzhai Member in the Sichuan Basin, the Lucaogou Formation in the Santanghu Basin, and the Ganchaigou Formation in the Qaidam Basin also have larger intra-source oil potential. They will all be important oil resources that support the long-term stable production of 2×10⁸ t of crude oil in China. In addition, the recoverable resources of China’s continental medium-low mature shale oil are mainly distributed in the middle-low mature shale strata with rich organic matter, such as the Chang 7 Member of the Ordos Basin and the Nenjiang Formation in the Songliao Basin^[3]. It is expected to realize effective exploitation through in-situ heating and conversion. If it can break through the “technical and economic barriers”, it will become a major replacement for petroleum resources in the future.

Future exploration and development in continental intra-source oil will still face many challenges: (1) In terms of basic geological research, shale strata have poor physical properties, mainly micro-nano pore throat systems. Under the evolution conditions of temperature field, pressure field and stress field, the coupling relationship between fluid and pore-fracture medium has not been clear. (2) In terms of evaluation criteria and evaluation system, the accumulation mechanism and major control factors of oil resources in shale strata have their own characteristics. For the evaluation of “sweet spot zones/intervals”, there has been still a lack of fine description, the drilling rate of sweet spots has been still lower, it is urgent to establish the evaluation parameter standards, methods and techniques of “new four properties”: reservoir, oiliness, fluidity and compressibility. (3) In terms of development theory and technology, continental intra-source oil accumulation has evident differences with marine shale oil. It can’t simply copy the existing horizontal well + volume fracturing technology. It still needs in-depth researches on phase state judgment, flow mechanism, development method, well layout method, development and production allocation, economic model, fracturing mechanism, and enhanced recovery technology, and it is urgent to develop a series of targeted “volume development” technologies. (4) In terms of engineering technology and engineering process, horizontal well drilling is characterized by slow drilling and completion speed, long cycle, and unadaptable well logging and mud logging technology and horizontal well monitoring technology, especially the fracturing technology has not yet been mature. It is still facing the challenges of “unsqueezed, unsupported, low flowback, and difficulty in stabilizing production”. (5) In terms of innovative system and mechanism, effective utilization and development, it is not yet adaptable to the effective development and utilization of low-grade oil resources, still facing challenges such as low single-well production, poor drilling time efficiency, long drilling cycle, and plan design to be optimized. (6) The core technology and equipment for in-situ conversion of medium-low mature shale oil has not yet been formed that it is in desperate needs to establish shale oil laboratories to consolidate the research foundation, to improve the evaluation parameters, to conduct timely field tests of shale oil conversion potential and technical and economic feasibility, to speed up the mastery of in-situ conversion of small wellbore, small well-spaced horizontal well drilling and completion, and super-long continuous heating and other core technologies and key equipment.

Exploration and development practices in China and abroad have shown that only extraordinary and revolutionary measures can achieve large-scale and efficient development of intra-source oil resources^{[1, 5, 39, 44-45]}. In accordance with the petroleum geological conditions and development actualities of intra-source oil in CNPC exploration areas, adhere to the principle of “first to explore easy resources and then difficult resources, concentrated exploration scale to increase reserves, and development tests to lead profitable development”, and implement the development strategy of “centralized construction, orderly advancement, and exploration and preparation”. Careful planning and hierarchical advancement will finally realize the “volume development” of two types of continental intra-source shale oil resources with medium-high maturity and medium-low maturity, thereby realizing large-scale and efficient exploitation (Fig. 6). The first level is to intensively establish the intra-source oil resources in the areas where resources are available and the primary development technologies are basically mature, including the Chang 7 Member of the Ordos Basin, the Lucaogou Formation in the core area of the Jimusaer Sag in the Junggar Basin, the Kong 2 Member of the Cangdong Sag in the Bohai Bay Basin, and Tiaohu Formation in the Santanghu Basin, so as to promote rapid production. At the second level, to progressively promote intra-source oil exploration in the areas with breakthroughs and certain development test foundations, including the Qingshankou Formation in the Songliao Basin, the Lucaogou Formation in the peripheral area of the Jimusaer Sag and the Fengcheng Formation in the Mahu Sag in the Junggar Basin, achieving integrated reserves and production growth. At the third level, to explore and prepare for intra-source oil with potentials, such as the Jurassic in the Sichuan Basin, the Shahejie Formation in the Bohai Bay Basin, and the Cenozoic in the Qaidam Basin, to well conduct resource evaluation and technical reserves. At the fourth level, to steadily promote the deployment and exploration of shale oil with low-medium maturity.

Large-scale petroleum resources generally exist in continental shale hydrocarbon source strata of China. Oil accumulation inside source kitchen is the accumulation of hydrocarbons retained in-situ or captured within the source rock, with large-scale resources. The three primary types (interlayer, hybrid sediment and shale) of “sweet spots” developed under the overall intra-source oil-bearing background are relatively richer in oil, better in physical properties, easier to reform, and have commercial value and great potential for development.

Through scientific and technological researches in recent years, basic research cognitions of continental intra-source oil accumulation, resource distribution, sweet spot evaluation, and development geology have gradually been deepened. The series of key exploration and development technologies concentrated on “sweet spot exploration” and “volume development” have been developed. The pilot test will continue to provide important scientific basis for the economic and effective development of intra-source oil accumulation, and the intra-source oil accumulation has shown a good theoretical and technical foundation and prospects for exploration and development in key exploration areas and strata.

The source rock-reservoir-caprock combination of intra-source oil accumulation is evidently different from conventional oil reservoirs, however, it is a brand-new accumulation system. At present, researches and explorations are still in infancy. Some geological cognitions may not be fully mature, and basic researches may not be fully intensive, such as the aspects of fine-grained sedimentary facies belts of shale, source rock-reservoir coupling mechanism of shale strata, hydrocarbon flow mechanism. The technology research and development may not be completely relevant, such as applicable development engineering technologies, low-cost factory-based operations, fracturing liquid effect. However, we firmly believe that, through continuous industrial practice, our cognitions will be further deepened, the technologies will be further improved, and China’s intra-source oil accumulation will be eventually in great development.

This article was written and researched with the help and support of CNPC Exploration and Production Branch, Science and Technology Management Department, various oil and gas field enterprises and scientific research institutes. We express our sincere thanks to them.