Petroleum exploration and development practices of sedimentary basins in China and research progress of sedimentology

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 37, Issue 4, August 2010 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2010, 37(4): 385–396.

RESEARCH PAPER

Petroleum exploration and development practices of sedimentary basins in China and research progress of sedimentology Sun Longde1, Fang Chaoliang1, Li Feng1, Zhu Rukai2,*, He Dongbo2 1. China National Petroleum Corporation (CNPC)，Beijing 100724, China; 2. PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China

Abstract: Based on the characteristics of recent discoveries in China, continental clastic reservoirs are the most important area for reserves and production growth. Significant progress has been made in exploration of carbonates reservoirs, for example, platform-margin reef complexes and platform interior reefs and banks. Volcanic reservoir exploration in sedimentary basins is feasible now. Mature oilfields with high water cut are predominant in the oil production in China. Some technological problems facing the stable production of mature oilfields have been solved by fine characterization of reservoirs, improving water flooding conditions, and EOR techniques. CNPC makes progress on studies of the sedimentary pattern of continental lacustrine basin shallow water delta, the origin and distribution of sandy debris flow, the mechanism and distribution prediction of deep favorable reservoirs, the sedimentary facies evaluation and reservoir prediction of low-porosity and low-permeability conglomerates, the lithofacies palaeogeography reconstruction of marine carbonates, the fine characterization of carbonates platform margins, the mechanism of carbonate reservoir superposition and rework, the origin classification of karst reservoirs, unconventional reservoir evaluation, reservoir improvement techniques, etc. These provide important theoretical and technical support for the exploration and development of oil and gas. Key words: exploration and development; sedimentology; reservoir geology; prospect

Practices in exploration and development of petroleum and other mineral resources in sedimentary basins have put forward many new tasks, such as the genesis interpretation and distribution pattern analysis of newly discovered oil and gas reservoirs, the high-precision lithofacies palaeogeographic reconstruction of sedimentary basins, the evaluation and prediction of deeply buried favorable reservoirs, the genesis mechanism and distribution pattern analysis of unconventional reservoirs, as well as the fine characterization and description of reservoirs. All of these have driven review of the petroleum exploration and development practices in sedimentary basins in the past and outlook of the directions in sedimentology development in the future[1,2].

1 Recent global practices in petroleum exploration & development and implications The reservoirs in oil and gas bearing basins in the world consist of clastic and carbonate rocks primarily, and volcanic and metamorphic rocks secondarily. In terms of the formations enriched with petroleum, most of the global oil and gas resources are reserved in the marine formations, of which the

recoverable resources are about 6 451×108 t, or 72% of the total recoverable resources; the recoverable resources contained in the continental formations are about 2 491×108 t，or 28% of the total[3−10]. In 2009, the proved oil reserves in the world were estimated at 1 855.04×108 t, up 0.89% year-on-year, and the proved gas reserves were estimated at 187.16×1012 m3, up 5.68% year-on-year[5,6]. The remaining reserves and production keep growing in recent years in the world, and the reserves-production ratio stays rather stable. Currently, the annual oil production of the world is about 40×108 t, and the reserves-production ratio remains at 45 or so; the annual gas production of the world is about 3×1012 m3, and the reserves-production ratio is as high as over 60. In 2008, the oil production of the world reached 36.5×108 t, up 0.8% over the previous year, and the gas production was around 30 501×108 m3, up 6.7% over the previous year. This is mainly attributed to the new advances and breakthroughs in petroleum exploration and development in the world in recent years[7−10]. Viewed from the characteristics of petroleum discoveries made in recent years, carbonate rocks are still very important places to find giant and super giant fields, and the marine area

Received date: 25 Mar. 2010; Revised date: 27 May 2010. * Corresponding author. E-mail: [email protected] Copyright © 2010, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

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has been the most important place to find giant fields. The new discoveries made in carbonate petroleum exploration are still concentrated in the Caspian Basin and the Middle East Gulf region. There are four major marine prospects in the world, including the Mexico Gulf, the Brazil Coast, the West African Coast and the North West Shelf of Australia (water depth exceeding 300 m). According to statistics, 18 big oilfields (of which the recoverable reserves exceed 5×108 bbl or 6 850×108 t) and 15 big gas fields (of which the recoverable reserves exceed 3×1012 ft3 or 850×108 m3) were discovered in these four deep water regions from 2000 to 2007, accounting for 42% of the total discovered resources in the same period in the world[11]. For instance, a number of giant fields have been discovered in Santos Basin in the west coast of Brazil since 2006: the recoverable reserves of Tupi Oilfield discovered in 2006, Carioca Oilfield, Caramba (1-SPS-51 discovery) Oilfield, Guara Oilfield, Jupiterr Oilfield and Lara Oilfield discovered in the 2007 are estimated at 50.07×108 bbl, 8.67×108 bbl, 9.34×108 bbl, 8.81×108 bbl, 5×108 bbl and 29.56×108 bbl, respectively[8,11]. The breakthroughs made in offshore petroleum exploration in recent years are attributed to the innovations in deepwater deposition theories such as the proposal of sandy debris flow theory and the research on the geometries of the deepwater sand bodies on the one hand, and benefit from the progress achieved in geophysical exploration and deepwater drilling technologies on the other hand. Remarkable discoveries made in foreland thrust belt petroleum exploration are mainly distributed in Zagros, the Andes and other regions, where the traps are primarily huge anticlines and blocks; a series of new discoveries are also made in exploring lithostratigraphic reservoirs in West Siberia, northern Venezuela, Maracaibo Basin, the North Sea Basin, and subtly exploring the mature areas. Others that deserve attention include exploration and development of tight sandstone gas, coal bed methane and shale gas. Rapid advances in engineering technology have already been achieved in these areas (Table 1)[9].

2 Practices of petroleum exploration and development in China Many sedimentary basins are developed in China, including 424 continental, 12 marine, 69 marine and continental superimposed basins, among which, small to mid-sized basins dominate. According to the latest resource evaluation results, China has about 430×108 t of onshore recoverable resources and 95×108 t of offshore recoverable resources[12]. Table 1

Unconventional gas production in major countries (by

the end of 2009) Country U.S. Canada

Tight sandstone gas (108 m3)

Coal bed methane (108 m3)

Shale gas (108 m3)

1 775

500

500

500

80

10

Australia

35

China

16

In recent years, China’s petroleum discoveries are mainly made in seven basins including the Songliao, Bohai Bay, Ordos, Sichuan, Tarim, Junggar and Qaidam basins. Ten oilfields each with the proved reserves exceeding 100 million tons are discovered lately, including the Gulong, Daqingzijing and Honggang-Daan oilfields in Songliao Basin, the Jidong Nanpu, Bozhong 25-1S and Penglai 19-3 oilfields in Bohai Bay Basin, the Xifeng and Jiyuan oilfields in Ordos Basin, the Luliang Oilfield in Junggar Basin and the Tahe-Lunnan Oilfield in Tarim Basin. Seventeen gas fields with the proved reserves exceeding 500×108 m3 are discovered, including the Xushen and Changling gas fields in Songliao Basin, the Sugeli, Daniudi, Shenmu and Zizhou gas fields in Ordos Basin, the Guang’an, Hechuan, Puguang and Luojiazhai gas fields in Sichuan Basin, the Tainan gas field in Turpan-Kumul Basin, the Tazhong 1, Kela 2, Di’na 2 and Dabei 1 gas fields in Tarim Basin, the Kelameili gas field in Junggar Basin and the Liwan 3-1 gas field in the South Sea Basin. Viewed from the characteristics of the petroleum discoveries, petroleum exploration in onshore clastic rocks are still the most important for increasing reserves and production. For instance, three oilfields, including Xifeng, Jiyuan and Huaqing, with the proved reserves ranging from 5×108 t to 10×108 t are discovered during exploring the Mesozoic lithostratigraphic reservoirs in Ordos Basin; exploration shows the Paleozoic lithologic gas reservoirs contain OGIP over 2.2×1012 m3. These achievements are attributed to many factors such as the establishment of the genesis models of sand bodies deposited in lake basins, the new understanding of reservoir distribution patterns, the analysis of genesis mechanism of tight reservoirs and evaluation of favorable reservoirs, the application of new exploration & development technologies including high resolution seismic acquisition in large area, seismic reservoir prediction under sequence constraint, full digital 2D seismic reservoir prediction, horizontal wells and fracturing stimulation techniques. Petroleum exploration in carbonate rocks has many discoveries in China in recent years. Significant discoveries are made in platform margin reef complexes, platform margin reefs and oolitic beaches. Three carbonate petroleum provinces including Tarim, Sichuan and Ordos have been found. Volcanic reservoirs in sedimentary basins have become a new field for petroleum exploration in China. Exploration of volcanic reservoirs has been carried out across the country. Specifically, activities of volcanic reservoir exploration have been conducted in the Bohai Bay, Songliao and Junggar basins since 2002, and a series of achievements and breakthroughs have been reached. These areas have become a highlight in China’s petroleum exploration. The deep layers in Songliao Basin and the Carboniferous formations in Junggar Basin have respectively become the 5th and the 6th continental gas provinces in China. The mature oilfields with high water cut are the main body in China’s oilfield development. Among the oilfields controlled by China National Petroleum Corporation (CNPC), the

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oilfields with water cut beyond 60% contribute 70% of the production. The continuous fine reservoir characterization research, the improvement of water flooding conditions and the progress in EOR technology have broken the bottle neck of stable production in mature oilfields. The recovery factor of the old oilfields which were put into production before 1998 has increased from 34.8% in 1998 to 38.1% in 2008, and the recoverable reserves of these fields have increased over 5 000×104 t annually. China’s oil production and gas production are 1.89×108 t and 775×108 m3, respectively in 2008, up 2.0% and 11.6% over 2007, respectively.

3 Advances in study on sedimentary reservoirs in the oil and gas bearing basins in China In recent years, CNPC has continually made new advances in the research on continental lake basin deposition and marine carbonate deposition, the evaluation of unconventional reservoirs, the study on reservoir sedimentology, the development of new techniques and technologies for reservoir sedimentology and sedimentary reservoir research, the improvement of reservoir stimulation and so on. These advances have provided important theoretical and technical supports for petroleum exploration and development. 3.1 New viewpoints and achievements in study on continental clastic rocks China is characterized by abundance of continental oil and gas bearing basins. China’s research level of land sedimentary reservoirs is in a leading position in the world. Systematic models of onshore sedimentary systems and sand body types have already been built[1,2,13−15]. In recent years, many innovative achievements have been made in the sedimentary pattern prediction of shallow water deltas in depression lake basins, the sedimentary pattern prediction of sandy debris flows in the deep water area of lake basins, the water-rock reaction mechanism and secondary pore development and distribution research, development mechanism and distribution pattern prediction of deep favorable reservoirs, the quantitative evaluation of diagenesis of sandy conglomerate reservoirs with low permeability and low porosity, as well as the prediction of the favorable reservoirs with low permeability and low porosity. These achievements have provided accurate basic geologic data for the division and evaluation of lithologic belts, and forcefully promoted the development of petroleum exploration. 3.1.1 Sedimentary patterns of shallow water deltas and sandy debris flows in deep water areas The clasolite filling patterns of continental lake basins can be classified into eight categories via analysis of the Mesozoic and Cenozoic lake basins in China[13,14]. Previously, the petroleum exploration practices focused on the reservoirs around the peripheries of lake basins, guided by the thoughts that sandstone reservoirs are not developed in the deep water regions of lake basins thus they are a forbidden area for petro-

leum exploration. However, in recent years, the in-depth petroleum exploration practices carried out in the continental lake basins have revealed that sand bodies with big gross thickness, large area and good physical properties are also developed in the centers of large scale depression lake basins. As a result, the scope of exploration has been continually enlarged and the reserves have been increased. Two types of sand bodies are developed in the lake basin centers, one type is of traction current genesis, including river, shallow water delta, lake current and density underflow sand bodies; the other type is of gravity flow genesis, including flood turbidites, slump gravity flow and sandy debris flow deposits. This knowledge has changed the viewpoint that the lake basin centers are dominated by mudstone and no effective reservoirs are developed there[13,14]. A whole new field for petroleum exploration is thus opened up. Through the outcrop observation and remote sensing image analysis of the typical shallow water deltas in lake basins in the world[16,17], combining the geologic research on the large Mesozoic and Cenozoic depression lake basins in China, including the Mesozoic Xujiahe formation in Sichuan Basin and the Mesozoic Yanchang Formation in Ordos Basin[18−20], the authors believe that the end distributary channels and the end crevasse fans are typical microfacies for shallow water deltas to develop in lake basins, the existence of open flow lake basins is an important condition for shallow water delta sand bodies to develop in the centers of lake basins, and the open flow channels exert important control on the distribution of sand bodies in the centers of lake basins. The latest results of research on the gravity flows of deepwater sediments in the world show that[21,22] there are few typical turbidites in deepwater sediments, and the sandy debris flows are important mechanism for thick sand bodies to form. The authors think, through comprehensive analysis of the Triassic Chang 6 Formation’s sandstones containing mud and gravels and the massive sandstones without any stratification taken from the Baibao area of Ordos Basin, that they are formed by sliding delta front sand bodies under external trigger force after these sand bodies accumulate rapidly, and are typical sediments of sandy debris flows. Under the effect of trigger force, the loose sand layers slide and collapse, and then slump, when a sand layer may break into many blocks from a whole unity, accompanied by large number of soft sediment transformation. As water flows in continually, the sand blocks break down and get roiled, forming lamellated debris flows and large area of tongue like sandy debris flow bodies on the slope of delta front platforms and the deep lake plains. A small number of turbidite sediments may be developed in the front or on the top of the debris flow deposition locations. The sandy debris flow sand bodies do not stretch far along the water flow directions (Fig. 1), and are important reservoirs in the deepwater areas of the lake basin centers (Fig. 1)[23,24]. The sandy debris flows in the region are of gravity flow genesis “without root”, which are dramatically different from the underwater fan sand bodies formed by the gravity flows “with root”.

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physical properties and control factors; step III, decide the distribution patterns of diagenesis, which is done through combining sedimentary facies, logging facies and seismic facies; step IV, assess comprehensively, during which the favorable reservoirs and diagenesis traps are predicted based on integrated mapping. Integrated mapping the diagenesis of the lower Xu 6 in the Guang’an region of Sichuan Basin indicates that, horizontally, the favorable diagenetic zones control the distribution of the belts with abundant petroleum resources and high productivity (Fig. 2). This is of great theoretical and practical implication for evaluation of petroleum reservoirs and exploration and development of tight sand gas reservoirs[26,27]. 3.1.3 Research on the genesis of effective deep to super deep clasolite reservoirs

Fig. 1 Plane distribution of sedimentary facies of Chang71 of Yanchang Formation in Heshui area of Ordos Basin

3.1.2 Diagenetic evaluation of tight sand reservoirs with low permeability Tight sand gas reservoirs exist in almost all petroleum provinces, and their resource potential is huge[25]. The tight sand gas reservoirs with low permeability are distributed widely in Sichuan, Ordos, Turpan-Kumul, Songliao, the south Junggar, the southwest Tarim, Chuxiong and the East Sea basins or regions in China and display as various types. The predictive evaluation of the “sweet spots” in thick sand bodies with low permeability is the key to exploration of petroleum reservoirs covering large areas. For example, the Xujiahe sand bodies in Sichuan Basin are generally thick 250−300 m, and the reservoir thickness is generally 50−90 m, accounting for 20%−30% of the total thickness. Therefore, how to find relatively favorable reservoir zones in the thick sand bodies covering large areas has become the key to exploration of such kind of reservoirs. Based on measurement of the paleotemperatures and anion concentrations of the clastolite inclusions in different layers and areas in the Upper Triassic Xujiahe Formation of Sichuan Basin, the law for anion concentration variation versus paleo-fluid evolution is revealed, the relationship between fluid evolution and genesis of tight reservoirs, and the relationship between them and the reservoir capability are further studied. In addition, the process of quantitative evaluation of the diagenesis of sandy conglomerate reservoirs and prediction of favorable reservoirs are determined: step I, decide the diagenesis sequence to determine the diagenesis environment and the diagenesis evolution process; step II, decide the types of diagenesis, which is done by analysis of cores, thin sections,

Scientific research and exploration practices related to deep petroleum have been increasing in recent years. For instance, USGS has ever carried out resource evaluation of the formations deeper than 4 572 m in the regions in its territory where petroleum was yet to discover; Russian geologists argue that when predicting the petroleum resources in Russia, the whole sedimentary formations (including covers) should be regarded as petroleum plays and prospects, rather than just the formations not exceeding 7 000 m in terms of burial depth[28]. In East China, deeper formations have increasingly become the petroleum exploration targets; in Tarim Basin of West China, reservoirs deeper than 5 000 m have been produced. What’s more, Well Dabei 3 in Kuqa Depression shows that good reservoirs are developed in the formations deeper than 7 000 m. In a word, there are various indications showing it is imperative to strengthen the deep oil and gas research and exploration. The core of deep petroleum exploration and research includes areas such as the phase state of deep oil and gas, the mechanism of deep reservoir development and preservation, as well as the conditions for deep oil and gas to accumulate. In-depth study on the genesis of effective deep reservoirs in sedimentary basins indicates that the genesis mechanisms for effective deep reservoirs to develop include: protected by the clay film, buried slow in the early and fast in the later stages, charged with hydrocarbon in the early stage, experienced secondary corrosion and tectonic faulting and fracturing. This has provided theoretical basis for discovery of deep oil and gas, and increased the exploration depth by nearly 2 000 m. 3.2

Marine carbonatites

The marine carbonatites in China are characterized by being old in era, deep in burial depth, mature in hydrocarbon source rocks, strong in reservoir heterogeneity, and complicated in oil and gas accumulation and distribution, which makes petroleum exploration facing a series of challenges[29]. Fortunately, achievements have been made continually in recent years in the fields such as the reconstruction of lithofacies and palaeogeography, the fine depiction of carbonate platform margins, the detailed description of the structures

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within the platforms, the mechanism of multi-stage superposition and transformation of carbonate reservoirs, the genesis categorization of karst reservoirs, the description and evaluation of heterogeneous carbonate reservoirs. These achievements have set a solid foundation for confirming hydrocarbon source rocks and distribution of effective reef flat reservoirs, evaluating oil and gas resource potentials and selecting favorable belts and zones, and promoted significant discoveries in carbonate petroleum exploration in China. 3.2.1 Reconstruction of lithofacies and palaeogeography and fine depiction of carbonate platform margins New advances have been achieved in the stratigraphic division and distribution of marine formations, the reconstruction of lithofacies and palaeogeography, and the distribution and evaluation of marine carbonate zone with high energy in the three large basins in China, including Tarim, Sichuan and Ordos Basins. The internal structures of the carbonate platform margins and the differentiation characteristics inside the

Fig. 2

platforms are depicted in detail, based on industrial mapping of the lithofacies and palaeogeography of the CambrianOrdovician formations in Tarim Basin [30,31] , the Permian-Triassic formations in Sichuan Basin[32,33], and the Cambrian-Ordovician formations in Ordos Basin[34,35] (Fig. 2). This has provided basic geologic data for analysis of the basin evolution and hydrocarbon source rock distribution, and further defined the distribution of favorable reservoirs, making the petroleum exploration extend to both platform margin reefs and inside platform reefs. 3.2.2 Genesis and distribution analysis of karst, reef and dolomite reservoirs Through analysis of the characteristics and genesis of marine carbonate reservoirs in Tarim, Sichuan and Ordos Basins, the authors think that, carbonate reservoirs are generally controlled by the multi-stage superposition transformation and quasi-syngenetic deposition - diagenesis, pores are developed in the early stage, the development of karst holes and

Superposition of Changxing sedimentary facies and reef flats in Sichuan Basin

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fractures in multi-stages is controlled by the tectonic-pressure coupling forces, the development of caves and pores in the deep formations is controlled by the reactions between fluid and rocks, but the development of effective reservoirs is not completely controlled by the burial depth. Based on such knowledge, reef flat, karst and dolomite reservoirs are classified into nine sub-categories, and a new karst genesis classification system is put forward, which categorizes the karst genesis into five types: karst developed by the weathered crusts of buried hills located on the core and top of the paleo-uplifts, karst developed between the core and the inside parts of the paleo-uplifts, karst developed by the deep undercurrents along the layers located on the slope and in the inside parts of the paleo-uplifts, karst developed by the vertical deep undercurrents formed by the downward seepage of fresh water in the atmosphere, and karst developed by the upward flow of hot fluid along the fractures. The presentation of karst developed by the deep undercurrents along the layers indicates a new exploration field. For instance, the exploration breakthroughs made in the Halahatang depression and Yingmai 2 structure located in the Tabei uplift of Tarim Basin have at least doubled the exploration range of karst reservoirs[36−41]. 3.3

Evaluation of unconventional reservoirs

Evaluation of petroleum resources in unconventional reservoirs such as volcanic oil and gas, coal bed methane, tight sand gas and shale gas reservoirs has been developed rapidly. Progress in volcanic reservoir studies is mainly displayed in the analysis of tectonic settings for volcanic development, the building of volcanic lithology and lithofacies models, the reconstruction of volcanic lithofacies and palaeogeography, the analysis of major factors controlling the reservoirs, the prediction of volcanic reservoirs by integrating gravity, magnetic, electric and seismic exploration, and the prediction of favorable reservoir distributions. Such progress has laid a foundation for evaluation of the potential of petroleum resources in volcanic reservoirs and evaluation of favorable volcanic reservoirs, and contributed to the continual breakthroughs in exploration of volcanic reservoirs in China[42,43]. According to the genesis, the pores in volcanic reservoirs can be divided into primary pores (air pores, intergranular pores and intercrystalline pores), secondary pores (dissolved pores and dissolved caves) and fractures (contracted fractures, burst fractures, sheered fractures and weathered fractures). The diagenesis has dual influences on volcanic reservoirs. The properties of the reservoirs are directly affected by the matching relationship between the filling and the dissolution in each diagenetic stage and the strength of the diagenesis. The weathering and leaching effect during the diagenesis is the key to improving reservoir properties. Meanwhile, the development of reservoirs is also impacted by the lithofacies, rock types, and so on. In summary, the volcanism, tectonic movements, weathering and leaching actions as well as the fluid action are the major geologic factors influencing and controlling the development degree of volcanic reservoirs.

All the volcanic reservoirs have experienced epigenesis during the period when they are formed, and the volcanic reservoirs can be categorized into two types: one is with weathered crusts, and the other is primary volcanic reservoirs. The volcanic reservoirs with weathered crusts are related to the structural unconformity surfaces formed by exposure and corrosion in a long period. Under the weathering and leaching actions exerted by the air and freshwater, the exposed volcanic rocks have formed weathered volcanic formations which are thick in vertical and distributed widely in horizontal. Secondary pores are developed in the high tectonic positions because of long period of weathering and leaching actions by air and fresh water; in contrast, most of the pores formed by dissolution are filled with chlorite and calcite in the low tectonic positions. The strength of weathering is the critical factor affecting the porosity of the reservoirs with weathered crusts. The primary volcanic reservoirs have not experienced exposure, weathering and leaching, since they have been buried deeply. As a result, most primary pores in such reservoirs have been preserved after the burial, the compaction, the corrosion and filling by hot fluid, and the dissolution by formation water and organic acid during the epigenesis period. Therefore, primary pores dominate the reservoirs pores. When it comes to coal bed methane reservoirs, research done lately indicates that structure uplift has different influences on the properties of the high rank coal bed methane reservoirs and the low rank coal bed methane reservoirs. The low rank coal bed methane reservoirs are dominated by matrix pores, while the high rank coal bed methane reservoirs are dominated by fractured pores. The structure uplift has distinct influence on the physical properties of the high rank coal bed methane reservoirs, reflected in reservoir pressure drop, cleat and fracture open, along with significant increase in fracture permeability. Dramatic uplift of high rank coal bed methane reservoirs can increase the permeability, causing a lot of methane loss, which is unfavorable to the accumulation of coal bed methane. In contrast, the structure uplift does not have strong influence on the physical properties of the low rank coal bed methane reservoirs. What’s more, for low rank coal bed methane reservoirs, the reservoir pressure drop because of the structure uplift can increase the speed of coal bed methane migration, which is favorable to the production of coal bed methane[44]. 3.4 Advances in research on reservoir sedimentology for development purpose To produce the remaining reserves in the mature onshore oilfields and enhance the recovery of the complex oil and gas fields, a lot of work has been done in fine and quantitative reservoir sedimentology, and significant achievements are made, displaying in the establishment of quantitative geologic knowledge base, the depiction of single sand bodies, the comprehensive application of logging seismic technology and progress in geologic modeling. Specifically, reservoir description techniques for 4 types of oil and gas reservoirs are de-

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veloped, which have played a very important role in efficient development of oil and gas fields and enhancement of oil and gas recovery. 3.4.1 Reservoir prototype models and geologic knowledge bases A reservoir prototype model refers to a physical reservoir model with high precision. Targeting at a certain sedimentary type, the model is set up under the control of dense enough data by using outcrops, modern sediments, dense well spacing and sedimentary physic modeling and experiments. With this model, a series of geologic knowledge bases are summarized, including the lithology and lithofacies knowledge base, the sedimentary microfacies knowledge base, the morphology and scale knowledge base of sand body assemblies, the physical parameter knowledge base and the parameter knowledge base of geologic statistics. The prototype model is a fine model for prediction of reservoirs of the same sedimentary type, and is able to effectively predict the lateral variation in reservoirs between wells. China started to research the prototype models of reservoirs during the ninth 5-year plan, and has set up prototype models and quantitative geologic knowledge bases for the braided river and fan delta sedimentary types. The knowledge bases include the width-thickness ratio, the length-width ratio, the probability distribution function and other parameters of different kinds of sand bodies. At the same time, a set of geologic modeling approaches have been developed and successfully used in the fine reservoir description and remaining oil production in the Daqing, Shengli, and TurpanKumul Oilfields[45]. 3.4.2

Description of single sand bodies

The description of single sand bodies under the guidance of theories on reservoir sedimentology and sand body geometry has become an effective technique for reconstructing the underground knowledge system in the secondary recovery of mature oilfields. Remarkable progress has been achieved in fine description of the single sand bodies of the Minghuazhen reservoirs with high water cut in Dagang Oilfield and production of the remaining reserves. The technique for identification and division of the lateral accretionary bodies within the meandering river point bar sand bodies is built by using seis-

Fig. 3

mic, well test and production performance data, based on the achievements of modern sediments, outcrops and laboratory sedimentary experiments. With the technique, the structure characteristics of each sand body is described; the method for modeling the internal structure of point bar sand bodies is developed (Fig. 3)[46]; the flooding patterns for five types of sand body groups are summarized; the inter-well connectivity and injection-production relations are analyzed once more; the fine geologic models for optimum grouping layers and wells are built. The water flooding recovery can be enhanced by over 7% due to the fine description of single sand bodies. 3.4.3

Four types of reservoir description techniques

As the reservoir description technology is used widely in China, a series of description techniques targeting at the characteristics of onshore reservoirs have been developed, which can be classified into four types[47]. (1) Reservoir description technique for the giant fields in stable sedimentary systems. Daqing Oilfield is a representative field to apply this technique. The structure in Daqing is rather simple, and the river delta deposition systems are complete. Fine analysis of the sedimentary microfacies is the key to description of such kind of reservoirs. By fully using the log data of close well spacing and the data of the inspection wells, along with fitting the sedimentary patterns and carefully characterizing the spatial distribution of sedimentary microfacies, the precision in prediction of the reservoir distribution can reach more than 85%. Since the fine division of the reservoir genesis units is completely applying the sedimentary microfacies to constrain the modeling, and the modeling is mainly definite, the multiple solution problem of stochastic modeling is prevented. (2) Reservoir description technique for complex block fields. Dagang Oilfield is such a representative field to use this technique. Dagang Oilfield not just has strong heterogeneity. What’s more, the faults of different levels intersect one another, making the reservoir description very difficult. With the fine interpretation of faults as the breakthrough point, plus using the high-resolution 3D seismic, the fine correlation of isochronous sedimentary units and the repeated fitting of production performance, the precision of fault interpretation can be enhanced greatly, and the small faults with 5−10 m of

Models of point bar structures of braided rivers[46]

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fault throws can be identified. In a word, with this technique, the in-depth understanding of the complex block reservoirs is realized. (3) Reservoir description technique for fractured reservoirs. Xinjiang and Yumen Oilfields are representatives of applying this technique. In fractured reservoirs, the reservoir permeability changes rapidly, influenced by fractures, and the distribution of fractures has poor regularity, making it difficult to predict the reservoirs. The breakthroughs of this description technique are: on the one hand, the fracture identification and prediction technology is formed, including description of the fracture distribution of outcrops and cores, identification of fractures by combining image logging and conventional logging, and prediction of fractures by 3D seismic; on the other hand, a 3D geologic modeling technology based on multi-data is developed, with which, geologic models are built by fully using various dynamic and static data, the neural network algorithm, the random interference interpolation, the Kriging interpolation, and the equivalent simulation of the spatial distribution of fractures. (4) Reservoir description technique for the “sweet spots”. Changqing Oilfield is a representative to apply this technique. As the effective reservoir layers, the “sweet spots” with higher permeability are scattered in the tight sandstones. Therefore, prediction of the units with higher permeability is the key to successful development. The steps of applying this technique are: first of all, to build the sedimentary diagenesis geologic model and the standard for identification of logging facies of the units with higher permeability; then, to analyze the overlapping patterns, contacts, dimensions and spatial distribution frequencies of the units with higher permeability by using the data of outcrops and wells with closer spacing, and modify them by using well test data; finally, to predict the “sweet spots” enriched regions with the geologic knowledge and seismic data, and set up 3D geologic models. 3.5 Advances in new technologies and new approaches for sedimentary reservoir research The means of researching sedimentary reservoirs is transiting from traditional sedimentology and petrology to multisciences and multi-information. Integration of the sedimentary patterns and the geophysical techniques can directly reveal and quantitatively describe the spatial distribution of the sedimentary bodies, and the advanced test means further strengthens the reservoir porosity evaluation and the fluid seepage characteristics research. 3.5.1 Seismic sedimentology and technique of fracture-cave-pore depiction of carbonate reservoirs Analysis of seismic sedimentology can not only disclose the basic characteristics inside the sedimentary systems, the transformation of paleotopography and paleogeomorphology, but also reveal the spatial distribution of various sedimentary systems in the isochronous frame and how they change as time goes by[48]. Notable fruits are harvested in applying the

seismic sedimentology technique in Daqing Oilfield and Bohai Bay Basin. For instance, a series of strata slices are made by using the Recon software for the sequences of Qi’nan Sag in Huanghua Depression. These slices provide continuous seismic images of the sedimentary system of Sha1 formation, and depict the distribution range of the braided delta sedimentary systems during different stages and the distribution locations of the underwater distributary channels and tributary bays. Such data provide reliable basis for exploring the thin, hidden reservoirs[49,50]. The technique for depicting the fractures, pores and caves in carbonate reservoirs is the integration of lateral prediction of the reservoirs via geologic, logging and seismic approaches and the description of pores, fractures and caves. The 3D distribution of the pores, fractures and caves are depicted accurately by well-seismic joint inversion and extraction of seismic attributes, under the guidance of geologic knowledge (Fig. 4). Pores, caves and fractures are developed in carbonate reservoirs, and the relations among the fractures, caves and micro-factures are rather complicated. Precise depiction and calibration of the factures, pores and caves in the reservoirs, together with the hydrocarbon detection and fluid identification techniques, has established a theoretical foundation for drilling trajectories. For example, the accurate knowledge of the carbonate reservoirs in the central Tarim Basin has not only ensured safe drilling footage, reduced the risks of drilling fluid loss, but also greatly raised drilling success. Since 2009, the general drilling success ratio has reached 83% in Tarim Basin, and the success ratio of drilling producing wells has reached 100%. 3.5.2

Laboratory of sedimentology outcrops

Research on the multi-information description of outcrops by many techniques has become a critical way of building the reservoir geologic models for exploration and development (Table 2)[51−54]. CNPC has primarily built the stratigraphic profiles of the typical outcrop sequences for three types of lake basins including the early Cretaceous depression lake basin of Luanping (Fig. 5), the late Triassic depression lake basin of Ordos and the late Triassic foreland lake basin of Sichuan, as well as the outcrop-downhole comparison models, by introducing new devices (e.g. the element capture instrument, the natural gamma spectrometer and the ground penetrating radar), and by setting up the outcrop sequence frameworks and geologic modeling of reservoirs. Based on the above work, the model of the sand body distribution within the cycle framework of the 3rd-order datum plane of the fault lake basins and depression lake basins are analyzed, the knowledge that continuous delta plain sand bodies enriched with oil and gas may overlay the cycle surface of the 3rd-order datum plane is presented, a number of visiting and training activities concerning the sequence stratigraphy of outcrops are initiated, and a platform is built for Chinese and overseas scholars to exchange ideas on continental basin sequence stratigraphy. These have actively facilitated the geologic research on lake basins.

Sun Longde et al. / Petroleum Exploration and Development, 2010, 37(4): 385–396

Fig. 4

Fractures, pores and caves of the Zhonggu 8 well unit in central Tarim Basin

Table 2 Name Element capture devices

Natural gamma ray spectrometers

Digital outcrop models (DOMs) Light Laser Detection and Ranging (LIDAR) or land laser scanner Ground penetrating Radar

Common devices used for building sedimentology outcrop laboratories Performance

Effect

Reference

To measure Ca, K, Na, Mg and Fe, etc

To build element geochemical profiles of the outcrops or cores

To directly display the K % and the U,Th ppm

To build the gamma curves of the outcrops and determine the sequence interfaces, to facilitate comparison of drilling and logging curves

[51]

To precisely measure and model the outcrop reservoirs

To build 3D geologic models of outcrop reservoirs

[52]

3D digital models with high resolution

To describe the structures of the reservoirs in detail

[53]

To directly get shallow layer seismic reflection sections of the outcrop profiles

To build the comparative models of seismic sections within the basin, and describe the geologic structures of the reservoirs in detail

[54]

Fig. 5 Skeleton within the cycle frame of the 3rd-order datum plane of the Sangyuan profile of the early Cretaceous rift lake basin in Luanping, Hebei

3.5.3 New techniques and approaches of sedimentologic experiments (1) Technique for quantitative evaluation of reservoir pore

structures. Analysis of the structures of micro-pores in reservoirs is one of the key points in reservoir study. Especially for the tight reservoirs with low permeability, the characteristics of the pore-throat structures are an important factor affecting

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Fig. 6

Quantitative reservoir pore structure evaluation

the productivity[55]. Philip H Nelson has classified the reservoirs into conventional sand, tight sand and shale reservoirs based on the pore throat dimensions. The diameters of the pore throats of conventional sand reservoirs are larger than 2 μm, the diameters of the pore throats of tight sand reservoirs are 0.03−2.00 μm, and the diameters of the pore throats of shale reservoirs are 0.005−0.100 μm[55]. CNPC has developed a technique for measuring the porosity of the reservoirs with low permeability by using Laser Scanning Confocal Microscopes and SEM back-scattering method. The above technique can be used to determine the types of micro-pores and their existing state. It is able to analyze core samples, and the smaller samples like debris. With this technique, the goal of analyzing the pore structures of the intervals without core sampling is reached, making the pore structure study transit from qualitative to quantitative and from macro to micro. It provides accurate data for reservoir research and important information and parameters for diagenesis research (Fig. 6). (2) Technique for diagenesis fluid analysis. In diagenesis research, the most important is to determine the conditions and ways of mineral-rock reaction, as well as the directions and channels of migration afterwards and the deposition locations. The technique is developed in two aspects: one is paleo-fluid recovery based on water-rock reaction, and the other is quantitative diagenesis research of basins. CNPC has developed a number of techniques including the one for laser Raman analysis of fluid inclusions, the one for stable isotope analysis by laser microscope sampling, the one for measuring the microelements in core samples via SEM energy spectrum-wave spectrum, the one for separation and purification of self-generated illites and the one of K-Ar dating. These techniques provide important supports for restoration of diagenesis fluid. For instance, analysis of the carbon and oxygen isotopes taken from the micro-sampling area of the dolomite reservoirs in Sichuan Basin shows that the carbon and oxygen isotopes taken from the dolomites of the oolitic beach in Well Luojia 5 are in the interval of evaporation environment, possibly reflecting the diagenesis of flow-back of tidal evaporation. Most of the samples taken from other wells

are in the interval of dolomite burial environment, probably related to the flow-back of brines. 3.5.4 Physical modeling and numerical simulation during deposition and diagenesis The experiments of water channel sedimentary modeling are developing to large-scale, refined and quantitative directions[56,57]. The simulation precision of the sand body distribution and the development of sedimentary systems including river delta, fan delta and braided river systems have been enhanced in recent years. For example, the experiments modeling the flood turbid flows indicate that parameters such as the slope and the altitude difference of the slope belt, the lake bottom terrain, the level of the lake water and the initial velocity have important effects on the turbid flow deposition. The silt content in the flood turbid flows determines the flow pattern and the sedimentary structure[58,59]. Physical modeling and numerical simulation of the reservoir diagenesis provides basic parameters for the quantitative characterization of reservoir diagenesis and the analysis of reservoir forming mechanism[60−62]. CNPC has successfully developed a set of giant equipment for modeling the diagenesis process and software for simulating the diagenesis. They can be used to simulate the diagenesis evolution of different proportions of clasolites and carbonatites, the vertical compaction/horizontal structure extrusion, the corrosion effects of single-phase/mixed phase and stacking phase acids (organic acid, HCO3−, H2S, HF, HCl, SO42− and so on), the cementation in diagenesis, the pressolution, the recementation and the tectonic disruption. They will provide theoretical bases for recover of paleo-rivers, analysis of reservoir genesis, evaluation and prediction of reservoirs. 3.5.5

New generation of reservoir description technique

The new generation of reservoir description technique includes the digital integration of various attributes of the 3D reservoir geologic bodies, the real time monitoring and simulation of reservoir fluids, the virtual reality display system, the automatic generation of the adjustment plans, and so on. The

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technical characteristics are full-digitization, real time performance monitoring and intelligent. 3.6

Reservoir stimulation techniques

3.6.1 Large-scale fracturing technique aiming at clastic reservoirs with low permeability A number of fracturing technologies have been developed, including the large-scale fracturing technique to generate long fractures in the sand reservoirs with large area but low permeability and porosity. It stimulates reservoirs and connects fractures primarily through sand fracturing. For instance, Well Long 35D-1 is fractured with the multi-layer fracturing technology. Three intervals are fractured at one time, and the daily production is tested at 12×104 m3 after fracturing. Significant progress has also been achieved in other fracturing techniques such as the appropriate scaled fracturing technology targeting the distributary channel sand reservoirs, the zonal fracturing technology, targeting the long intervals with multi-thin layers, to enhance the recovery percent of the sub-layers, and the water sensitivity resisting fracturing liquid technique with low damage which targets the strongly water-sensitive sand reservoirs with low permeability. 3.6.2 Section-selective acid fracturing technique to stimulate fracture-cave-pore systems of carbonate reservoirs The section-selective acid fracturing technique to stimulate the fracture-cave-pore systems in carbonate reservoirs can generate many artificial fracture systems through implementing large scale acid fracturing operations, so as to improve the flow capacity of the reservoirs around the bore holes and enhance the individual well productivity. For instance, the daily production of Well 62-7H in Tarim Basin has been raised from 2.86 t before acid fracturing to 208 t after acid fracturing. It produces gas 14.7×104 m3 per day after fracturing. 3.6.3

Technique for stimulating volcanic reservoirs

The permeability of volcanic reservoirs is very low because they are buried deep and their lithology is tight. What’s more, the oil bearing intervals in volcanic reservoirs often cross large spans, where fractures and dissolved caves are developed, resulting in enormous oil loss. Therefore, it is difficult to obtain economic productivity by conventional production approaches, and fracturing stimulation is indispensable. In recent years, a series of fracturing assisting technologies have been developed including the technique for building the fracturing breaking and extending models, the technique for on-site (the test and fracturing sites) quick interpretation, the high temperature fracturing fluid systems, the technique for fracture control and extension in hydraulic fracturing, as well as other assisting techniques including the gas layer protection, the borehole evaluation, the effective fracturing and production with horizontal wells. For example, 138 well intervals (including producers) drilled in the volcanic reservoirs in

Xushen gas field, Daqing, have been fractured in the past three years, the success ratio of fracturing operations has been enhanced from lower than 40% to over 90% at present. Specifically, the horizontal section of Well Shengping 1 in Daqing Oilfield is 500 m. It produces gas 55.5×104 m3 per day, four times that of the neighboring vertical wells. In addition, 20 layers in 15 wells of the deep volcanic reservoirs in Jilin Oilfield are fractured, and the success rate is 90%.

4 Reflection and outlook of the oil and gas potential and the direction of sedimentologic development The deepening of sedimentologic research is taking an increasingly important role in petroleum discovery, reserves increase and petroleum production. Meanwhile, the continual extension of exploration and development fields will definitely push forward the innovation and advancement of the sedimentologic theories. From the perspective of the remaining petroleum resources in the world, the total amount of the conventional oil and gas resources is abundant, but the recovery percent is not high at present; the unconventional resources are also rich, and have huge potential for reserves and production increase with the advancement of technologies. Hence they are noteworthy. For instance, some American experts state that the shale gas can be produced for 100 years[9]. Taking China’s exploration and development demands into consideration, the authors think the following aspects are the key directions for reservoir research: (1) Continental lithologic reservoirs will remain the principal target for exploration in the long future. Low porosity and low permeability are the characteristics of such reservoirs. To promote fine exploration and effective development of the lithologic reservoirs, research on the 3D sedimentary systems of the whole basin, the reservoir sedimentary mechanisms, scales, dimensions, distributions and the quantitative evaluation of diagenesis facies as well as the fluid media systems should be strengthened. (2) Continual breakthroughs in marine carbonate exploration indicate good prospects. What needs to be enhanced includes the prediction of high frequency sequences and effective reservoirs, the prediction of sedimentary microfacies and high energy zones, the mechanisms for deep paleo-reservoirs to form and to be preserved, the mechanisms of dolomite genesis and the genesis, the distribution and quantitative characterization of fractures-pores-caves in carbonate reservoirs. Particularly, since the early Mesozoic formations dominate the carbonate reservoirs in China, integrated research on sedimentation, diagenesis and transformation should be enhanced. (3) Offshore exploration is a new field. China does not have very much offshore exploration experience. To promote breakthroughs in offshore exploration, study on the shelf and deepwater sedimentary systems and reservoir characteristics should be accelerated. (4) Petroleum exploration goes deeper and deeper. New

Sun Longde et al. / Petroleum Exploration and Development, 2010, 37(4): 385–396

discoveries are continually made at the burial depth of 6 000− 7 000 m. Porosity evolution, preservation mechanism and distribution patterns of the deep reservoirs should be strengthened. (5) To facilitate exploration in the unconventional new field, the mechanisms of unconventional reservoirs (including tight sand, coal bed methane, shale, volcanic and matrix reservoirs etc.) should be conducted and strengthened, so do the evaluation of such reservoirs. (6) To maintain stable production in mature fields, fine and quantitative reservoir characterization techniques should be continually promoted, especially the description and analysis of the physical parameter variation and the big throat systems which impact the balance of injection-production. For the reservoirs with low permeability, quantitative description of the pore structures of multi-porous media and their impacts on fluid flow should be strengthened. (7) Application of multidisciplinary in reservoir research should be continually promoted. Study on the geophysical prediction and logging evaluation techniques under the guidance of sedimentary patterns should be enhanced to improve the exploration and development efficiency. (8) Laboratory evaluation of the pore structures in reservoirs with low permeability and ultra-low permeability, as well as the physical modeling of sedimentation and diagenesis should be strengthened, and a national reservoir database should be built.

Acknowledgements We thank Professors Zou Caineng, Gu Jiayu, Yuan Xuanjun, and Shen Anjiang for their discussions and help during the writing of this paper. Thanks also go to Drs. Zhang Xingyang and Wang Lan for their help.

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Zou Caineng, Tao Shizhen, Yuan Xuanjun, et al. Global importance of “continuous” petroleum reservoirs: Accumulation, distribution and evaluation. Petroleum Exploration and Development, 2009, 36(6): 669–682.

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