The Newly Discovered Yanchang Gas Field in the Ordos Basin, Central China

Journal of Earth Science, Vol. 28, No. 2, p. 347–357, April 2017 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-015-0661-5

Hairen Gao, Weili Wang* Oil and Gas Exploration Company, Yanchang Petroleum (Group) Co., LTD, Yan’an 716000, China Hairen Gao: http://orcid.org/0000-0001-5545-2782; Weili Wang: http://orcid.org/0000-0002-3435-3846

ABSTRACT: The Yanchang gas field is located in the Ordos Basin of central China to the southeast of the Yishan Slope, covers an exploration area of 2.6×104 km2 and has approximately 3.5×1011 m3 of gas reserves. The gas field is dominated by lithologic gas reservoirs but also has a few structural gas reservoirs. Sand bodies were deposited in the Carboniferous Benxi Formation around offshore barrier isl-

ands and in distributary channel fillings in the delta front of the P1s2 and P1s1 divisions of the Permian Shanxi Formation. The P2h8 division of the Shihezi Formation contains the main reservoirs. The depths of the reservoirs are between 1 970 and 3 500 m. The Yanchang gas field can be classified as a typical tight sandstone gas reservoir filed because its porosity is lower than 10% and permeability lower than 1 mD. The discovery and development of the Yanchang gas field has led to a great increase in total natural gas reserves in the Ordos Basin. Its exploration has improved methods of locating large gas fields in deep-water depositional environments in the south part of the basin. KEY WORDS: Yanchang gas field, Ordos Basin, Central China, tight gas reservoir, lithologic reservoir, gas in coal-bearing strata.

0 INTRODUCTION between 5 000–10 000 m (Zhao et al., 2014; Liu et al., 2006; Li, The Yanchang gas field is located in the southeast part of 2004; Zhang, 2004). Tectonic activity in the basin is primarily the Yishan slope of the Ordos Basin adjacent to the Jinxi fold controlled by the Xingmeng trough on its northern margin, the belt, and has an exploration area of 2.6×104 km2 (Fig. 1). Ex- Qinqi trough on the southern and southwestern margins, and ploration of this gas field began in 2003 and continued until the expansion, subduction, and reduction of the Helan aulaco- 2013. Approximately 3.5×1011 m3 proven total capacity of nat- gen (Xu et al., 2006; Yang, 2002; Pu et al., 2000). The geologi- ural gas reserves occur in tight Upper Paleozoic Carboniferous cal history covers five major stages, i.e. the Mid-Late Protero- and Permian sandstone gas reservoirs. Yanchang therefore con- zoic aulacogen, an Early Paleozoic shallow-sea platform, Late stitutes another large gas field located in the north of the Ordos Paleozoic coastal plains, a Mesozoic inland lake basin, and Basin in addition to the Sulige gas field. Many studies have Cenozoic surrounding faults (Zhao J Z et al., 2012; Yang, et al., been published about the geological conditions and distribution 2005). The current tectonic configuration of the basin was con- regularities of gas reservoirs in the Ordos Basin but most of trolled by Jurassic Mid-Yanshanian tectonic movements, and them have concentrated on gas fields in the northern area (Fu et the basin generally exhibits a stable development pattern during al., 2010; Fu et al., 2008), but only a limited number of studies its geological evolution (Zhao et al., 2010; Zhang, 1982). The have investigated the Yanchang gas field which is in the south- basin may be sub-divided into six tectonic units: the Yimeng east part of the basin (Wang et al., 2006). This paper describes uplift, the thrust belt on the western margin, the Tianhuan de- the geological characteristics and gas production of the Yanc- pression, the Yishan slope, the Jinxi fold belt, and the Weibei hang gas field, its exploration history, mode of formation, se- uplift (Fig. 1a). In the Upper Paleozoic the basin was controlled dimentary environment, structure, and reservoir characteristics. by tectonic and sedimentary events, and filled by the Upper Carboniferous Benxi, Permian Taiyuan, Shanxi, Lower Shihezi, 1 GEOLOGICAL SETTING Upper Shihezi, and Shiqianfeng formations, while Silurian, The Ordos Basin is a large multi-cycle cratonic basin lo- Devonian, and Lower Carboniferous formations are absent (Fig. cated in central China and occupies a major part of the North 2). China Craton. It contains a thickness of sedimentary rocks Hercynian movements were superimposed on a Caledo- nian collisional uplift in the Ordos Basin whose influence con- *Corresponding author: [email protected] tinued until the Late Carboniferous. Weathering and erosion © China University of Geosciences and Springer-Verlag Berlin occurred during intervals as long as 1.5–1.8×108 a (Zhao J Z et Heidelberg 2017 al., 2012). A carbonate erosion surface developed within the Ordovician Majiagou Formation. During deposition of the Late Manuscript received November 8, 2014. Carboniferous Benxi Formation, the Ordos Basin underwent its Manuscript accepted May 15, 2015. second Paleozoic transgression since the Ordovician (Chen

Gao, H. R., Wang, W. L., 2017. The Newly Discovered Yanchang Gas Field in the Ordos Basin, Central China. Journal of Earth Science, 28(2): 347–357. doi:10.1007/s12583-015-0661-5. http://en.earth-science.net 348 Hairen Gao and Weili Wang

Figure 1. (a) Tectonic subdivisions of the Ordos Basin and location of the Yanchang gas field; (b) Map of the Yanchang gas field including discovered gas bearing areas. Structure contours on the top of the Taiyuan Formation. A-A’ cross section shown in Fig. 4. et al., 2006; Zhang, 2005; Guo and Liu, 1999). A central pa- During Middle Permian Lower Shihezi times the sedimentary leo-uplift formed in the Early Ordovician and divided the Or- pattern inherited several traits from Shanxi times accompanied dos Basin into two marine areas in the east and west that con- by strengthened regional tectonic activity. A northern prove- trolled the distribution of sedimentary environments throughout nance region continued to uplift, and abundant terrigenous Benxi times (Deng et al., 2011; Huang et al., 2005; Xie et al., debris was deposited by rivers. This shallow water delta system 2005). The North China Sea migrated from east to west and rapidly pushed southward, which resulted in the southward from east to north during Benxi times in the eastern area of the migration of the upper delta plain-facies area. During Upper Ordos Basin, so that deposition overlapped westward and Shihezi and Late Permian Shiqianfeng times, tectonic uplift northward to form a wedge body that is thick in the east and forces weakened, and a lacustrine sedimentary system ex- thin in the west. The sedimentary environment was dominated panded northward. Thus the basin evolved from an Early Per- by a tidal flat-lagoon-barrier island-shelf sea (Qu et al., 2011; mian offshore lake basin to an inland lake basin. The sedimen- Wang et al., 2006; Zhao et al., 2006). During Early Permian tary environment completely converted to a continental system, Taiyuan times there was continuous regional deposition in the characterized by the development of rivers, deltas, and lacu- basin and the sea expanded from the east and west toward the strine sedimentary systems (Jiang et al., 2011; Xiao et al., Central Paleo-uplift and north edge of the basin. Tidal flats, 2008). lagoons, and onshore deposition overlapped the Ordovician paleo-erosion surface on the Central paleo-uplift, submerging it 2 EXPLORATION HISTORY AND FIELD DISCOVERY so that the east and west sides formed a unified sea area (Guo et Exploration for natural gas in the Ordos Basin began in al., 2012; Chen et al., 2010; Xi et al., 2009). Deposition was the 1950s and predicted complete natural gas reserves of still partly controlled by the Central Paleo-uplift, and the east- 15.2×1 012 m3 including a tight sandstone gas reserve with a ern region of the basin was dominated by epicontinental marine volume of 10.4×1 012 m3. So far six tight sandstone gas fields sediments. During Early Permian Shanxi times, the sea rapidly with reserves greater than hundreds of billion cubic meters have retreated from the east and west sides of the Ordos Basin be- been discovered, Sulige, Wushenqi, Daniudi, Yulin, and Mizhi, cause of overall uplift of the North China platform and signifi- all located in the northern and eastern areas of the Yishan slope. cant changes in the regional tectonic environment and sedi- Cumulative geological reserves of tight gas are approximately mentary pattern occurred (Yu et al., 2012; Chen et al., 2011; 3.5×1 012 m3 and account for 84% of the total proven natural Xing et al., 2008; Peng et al., 2006). The basin evolved from an gas in the basin (Guo et al., 2016; Jia et al., 2012; Fu, et al., 2010; epicontinental sea basin to an offshore basin, and the sedimen- Zhang and Zhu, 2008; Xiao et al., 2005). During the past several tary environment evolved from marine to continental facies. years, the Yanchang Petroleum Company (YPC) has

The Newly Discovered Yanchang Gas Field in the Ordos Basin, Central China 349

Figure 2. Upper Paleozoic sequence in the Ordos Basin. explored the new Yanchang gas field in the southeast part of the but did not yield commercial flow. Thus, exploration in the Yishan slope and shown that the entire basin contains abundant southeastern region of the Ordos Basin was temporarily aban- natural gas. doned. PetroChina’s Changqing Petroleum Company and Sino- YPC changed their exploration strategy early in the 21st pec’s North China Bureau conducted extensive research in the century, after comparison with the Sulige and Daniudi gas northern basin and successfully made revolutionary advance- fields in the north of the basin and further exploration expe- ments during exploration in the last 50 years (Zhang et al., rience. The company’s exploration targets transitioned from 2012; Zou et al., 2012; Yang, et al., 2008). The earliest explora- structural reservoirs to lithologic reservoirs. The company tion of the Yanchang gas field began in 1948 when an explora- therefore changed its exploration plan from a weathering crust tion well (Well YS1) was drilled by the YPC and passed in the Lower Paleozoic Majiagou Formation to sandstone re- through the Upper Paleozoic with no indications of exploitable servoirs in the Upper Paleozoic. The first exploration well quantities of natural gas. It was generally believed afterwards (Well YQ1) was drilled in 2003 in the center of the Ordos basin that reservoirs had not developed in the Upper Paleozoic delta and yielded low productivity gas flow from the Shiqianfeng front or in the Lower Paleozoic Majiagou Formation catchment Formation of 6 516 m3/day. After a fuller evaluation of natural basin in this area. Geological research could not be conducted gas potential throughout the area, wells YQ2 and YQ4 were because of the steep topography of the ground surface. Conse- drilled in Yanchang and Yanchuan Counties respectively. Well quently, natural gas exploration did not conduct in this area. In YQ2 yielded commercial flows of 30 799 m3/day from the 3 the 1980s, the Changqing Petroleum Company explored the Permian Shanxi Formation P1s2 and 15 996 m /day from the Fuxian and Yichuan areas in the southern region of the basin. Shihezi Formation P2h8 respectively. This result indicated that Wells FT1 and YT1, targeted the Majiagou Formation reservoir large gas fields could be found in tight sandstone reservoirs on

350 Hairen Gao and Weili Wang the Upper Paleozoic delta front. This discovery formally found to be 21.4 m by logging at a burial depth of 2 682 m (Fig. opened the exploration process in the Yanchang gas field. 3a). The lithology at this site is light gray medium sandstones, The YPC overcame tremendous difficulties, such as the and the absolute open flow (AOF) of gas was 1 288 703 m3/day. complex surface structure of the Loess Plateau area, significant Well Y128 was drilled and completed near Well Y127. Logging horizontal thickness fluctuations, the presence of low-velocity this well showed the thickness of the C2b gas-bearing reservoirs layers, poor seismic record quality (with weak energy and low to be 4.7 m, and its burial depth 2 651 m (Fig. 3b). The lithol- signal-to-noise ratios), and poor continuity of the targeted layer ogy at this site is light gray fine sandstones, and the AOF is 100 in cross-sections by applying high-resolution crooked line sur- 197 m3/day. It was thus concluded that this area is a veys in valleys, and using a multiple-coverage technique, op- gas-bearing region with multiple gas-containing reservoirs. By timum excitation approach, and refraction static correction of the end of 2014, the Yanchang gas field was predicted to con- two-dimensional seismic observations during 2006–2007. The tain approximately 350 billion m3 of known reserves. The

Carboniferous C2b and Shanxi formations P1s1 were confirmed Shanxi Formation P1s2 and Shihezi Formation P2h8 reserves to have superior reservoir development, based on previous especially accounted for 40% and 25% of total reserves respec- drilling experience and geological evaluations combined with tively, and are the main gas-bearing reservoirs in the Yanchang well logging and seismic data. The Permian Shanxi P1s2 and gas field. Although the Benxi Formation gas reservoir only Shihezi P2h8 sandstone formations also exhibited reservoir accounts for 17% of the total reserves, this formation contains development. sand layers with comparative small average thicknesses and In 2008 Well Y127 was completed in Yanchang County. relatively high average production (Table 1).

The thickness of the P1s2 gas-bearing reservoirs in the well was

Figure 3. Well logging curves of Well Y127(a) and Y128(b) in the Yanchang gas field, Ordos Basin.

The Newly Discovered Yanchang Gas Field in the Ordos Basin, Central China 351

Table 1 Primary gas layer parameters of the Upper Paleozoic reservoirs in the Yanchang gas field in the southeastern Ordos Basin

Gas reservoirs Reservoir thickness AOFs of the individual wells Relative density Porosity Permeability (mD) Methane content (%) (m) (m3/d) (%) Min–Max

Average 4–34 746–40 347 0.57–0.75 1.6–13.2 0.01–394.1 52.2–97.0 P2h8 10.2 15 978 0.60 6.1 1.1 90.4 4–32 2 143–41 441 0.57–0.82 1.2–11.9 0.008–78.3 66.2–97.5 P1s1 7.7 15 720 0.59 5.1 1.0 94.3

2–24 200–1 288 703 0.56–0.89 1.1–14.2 0.003-192.9 52.2–99.0 P1s2 6.8 77 144 0.61 4.5 5.5 93.2 2–29 3 065–573 041 0.55–0.79 1.1–8.6 0.01–104.0 70.18–99.1 C2b 4.6 68 439 0.59 4.4 1.3 95.0

3 STRUCTURE AND TRAP DEFINITION 1). The AOF variation between each well is relatively high and

The overall structure of the Ordos Basin is higher in the the AOF variation of single wells between P1s2 and C2b is rela- east and north and lower in the west and south. The Yishan tively high. The relative density of natural gas is approximately slope is the main tectonic unit of natural gas enrichment (Fu et normally 0.60 and the volume proportion of methane generally al., 2008) and accounts for more than half of the main area in greater than 90% (e.g. 95.0% in C2b). The porosity of the gas the basin, dipping gently west at less than 1º. The Yanchang gas reservoirs decreases from top to bottom and the permeability in field is located southeast of the Yishan slope and structural the P1s2 reservoir is higher than those of the other reservoirs traps are not well developed except for a few small, (Table 1). No gas-water interfaces have been detected in the gas nose-shaped uplifts that occur sporadically in the eastern region reservoirs, and most wells can only generate commercial gas of the gas field (Fig. 1b). flows after fracturing transformation. During the Upper Paleozoic, the basin was filled with se- quences of transitional facies and fluvial-deltaic facies. Mul- 4 STRATIGRAPHY AND SEDIMENTARY FACIES tiple sandstone layers overlap from bottom to top (Yang et al., Distribution of Upper Paleozoic reservoirs in the Ordos 2012; Zhao Z Y et al., 2012; Zhou et al., 2012). Because the Basin is controlled by sedimentary facies, which has a signifi- heterogeneity of the reservoirs is relatively high and lateral cant influence in predicting sedimentary environments of gas continuity of lithology and physical properties is poor, litho- fields, placement of the wells and development of gas fields logic traps are distributed over a large area (Zhu et al., 2008; when exploring for reservoirs (Zhao et al., 2005; Arthur et al., Zhang et al., 1997). The Yanchang gas field contains many 2001). This paper has reorganized the classification of the se- stacked gas-bearing formations, of which sandstone reservoirs dimentary environment in the main gas reservoirs of the Yanc- of the Carboniferous Benxi, Permian Shanxi (P1s2 and P1s1), hang gas field based on the distribution of sand bodies in the and Shihezi formations (P2h8) are the main gas-producing re- main reservoirs throughout the area, incorporating data from servoirs (Fig. 4). In several areas, natural gas is found in the outcrops, rock cores and well logging. karst weathered crust of the Ordovician Majiagou Formation. The Yanchang gas field developed a set of barrier isl- Permian sand bodies are primarily located in the sand-body and-tidal flat-lagoon marine depositional systems in Late Car- development area on the eastern Mizhi delta front of the Yishan boniferous Benxi times (Fig. 5). Its eastern side was primarily slope, far from their provenance area in the north. Underwater dominated by barrier island deposits with the Zhi- distributary channels were relatively small and sinuous, and dan-Ganquan-Fuxian line as a boundary, and its western side therefore multiple thin fine sand body belts accumulated in by tidal flat and lagoon deposits. Barrier islands distributed as underwater distributary channels in this region. Fine-grained bands along the northwest-southeast direction throughout the lithologies such as inter-distributary bay siltstone, argillaceous area provided the main sedimentary environment for sand body silty sandstone, and mudstone developed near channel sand development during Benxi times. Several areas in the east bodies to form unilateral plugs and up-tilting caps. The under- formed a contiguous distribution on this surface because of water distributary channels that developed following the depo- multi-episodic stacking of barrier-island sand bodies. Black sition of P1s2, P1s1, and P2h8 primarily penetrated shallow lacu- mudstone, carbonaceous mudstone, and thin coal seams depo- strine muddy shore sediments or occurred in up-tilting and sited in the lagoon environment are also important hydrocarbon lateral directions and were covered by inter-distributary muddy source rocks, forming an effective lateral occlusion for the sand bay sediments to form unilateral plugging traps with up tilted bodies surrounding the barrier islands and tidal flats. caps (Fig. 4). The Permian Shanxi Formation is divided into P1s1 and

The gas-producing reservoirs in the Yanchang gas field are P1s2; the Shihezi Formation is divided into the Upper and Low- primarily distributed at depths between 1 970 and 3 500 m and er Shihezi formations, which are further divided into four sec- are generally deep in the west and shallow in the east because tions (Fig. 2). In stages P1s2, P1s1, and P2h8, the main of the influence of terrain. The average thickness of the P2h8 gas-bearing reservoirs were primarily responsible for deposi- formation is the greatest of the gas-producing formations (Table tion at the delta front, and sand bodies in underwater

352 Hairen Gao and Weili Wang

The Newly Discovered Yanchang Gas Field in the Ordos Basin, Central China 353 distributary channels are the main natural gas reservoir sites in distributary channels spread relatively widely from south to the Yanchang gas field. Early in Permian times during overall north. The P1s2 part of the vertical section is primarily charac- uplift of the North China Block, the sea rapidly retreated and terized by inter-bedding of gray-white and light gray fine sand- the sedimentary environment transitioned from marine to con- stone, medium sandstone, and small amounts of siltstone, tinental facies, differences between the east and west regions gray-black mudstone, and sandy mudstone with variable thick- disappeared, while at the same time the differences between the nesses (Fig. 2). The sandstones developed a trough-shaped north and south regions strengthened. Zhidan, Fuxian, Yichuan, cross bedding structure, ripple bedding, deformation bedding, and Luochuan became the sedimentary centers in the basin and and erosional surfaces with occasional worm holes and carbo- developed shallow lacustrine sediments. Other areas developed nized plant debris (Chart I-a–e) but the mudstones only devel- sediments on the delta front (Fig. 5). During P1s2 times, the oped horizontal bedding. The shoreline area of a shallow lake Yanchang gas field mainly developed sedimentary micro-facies that arose in P1s1 and P2h8 times was slightly smaller compared in underwater distributary channels, inter-distributary bays, to its shoreline area in P1s2 times. Multi-episodic swings of mouth bars, and delta-front sand sheets. Narrow underwater linked underwater distributary channels in a north-south

Figure 5. Sandstone isopach maps (in meter) and interpreted sedimentary facies of the P2h8, P1s1, P1s2, and C2b members in the Yanchang gas field.

354 Hairen Gao and Weili Wang direction resulted in the contiguous distribution of sand bodies nescence, and scanning electron microscopy of samples from 20 on the plane and inter-distributary bays and mouth bar mi- wells. We also performed statistical analyses of physical proper- cro-facies were not developed (Figs. 5a, 5b). P1s1 sediments ties for 4 311 sample points from 213 wells in the area. We can primarily consisted of inter-beddings of gray-black and black thus describe the structure and characteristics of reservoirs in mudstone, silty mudstone, and argillaceous siltstone of varying four main intervals within the gas field. thicknesses accompanied by thin layers of fine sandstone. The statistical analyses indicate that the lithology of the

Mudstone developed horizontal bedding with a few plant fos- P2h8-C2b in the Yanchang gas field is dominated by lithic quartz sils (Chart I-f). Argillaceous siltstone developed sedimentary and quartz sandstone (Fig. 6). There are many different types of structures with trough-shaped and ripple cross bedding (Chart interstitial materials, such as clay minerals, siliceous cements,

I-g). In P2h8, sedimentary sand bodies were thicker compared and carbonate cements (Fig. 7). In P2h8 and P1s1 times, inter- with other strata due to lack of provenance supply and uplift of granular materials were dominated by lithic quartz and lithic the northern region of the basin (Table 1). In the vertical direc- sandstones with well sorted and rounded medium-coarse par- tion, this region is primarily characterized by inter-bedding of ticles ranging from 0.3–1.2 mm. Average interstitial content is gray-white to light gray fine sandstone, siltstone, dark gray 10.6% (4.0%–33.1%) in lithic quartz sandstones and 11.9% mudstone, and silty mudstone of varying thicknesses. The (1.2%–35.3%) in lithic sandstones predominantly chlorite, hy- sandstone developed a sedimentary structure with dro-mica, siderite, and ferro-calcite. The lithology of P1s2 is trough-shaped cross bedding, tabular cross bedding, an erosion dominated by quartz and lithic quartz sandstones with lithic surface, and parallel bedding (Chart I-h–i). sandstone development in several areas. The particles are mainly medium-coarse (0.25–1.0 mm) with relatively good sorting 5 RESERVOIR ARCHITECTURE AND PROPERTIES and rounding. The average interstitial material content of the The performance of a sandstone reservoir is directly af- matrixes is 6.1% (1.3%–38.0%) and dominated by hydromica, fected by material composition, pore structure, and physical silicon, ferrodolomite, and siderite. Because C2b was located in properties (Wang et al., 2008). We have gathered a great deal of a shallow shore sedimentary environment and subjected to relevant data for this paper, observing outcrop cross-sections in long-term erosion and wave action, the structural and composi- the field, and examining casts and thin sections, cathodolumi- tional maturity of the sandstone was relatively high, and the

Figure 6. Triangular diagrams of mineral compositions of Upper Paleozoic clastic rocks. 1. Quartz sandstone; 2. feldspathic quartz sandstone; 3. lithic quartz sandstone; 4. feldspar lithic quartz sandstone; 5. arkose; 6. lithic arkose; 7. feldspathic lithic sandstone; 8. lithic sandstone.

The Newly Discovered Yanchang Gas Field in the Ordos Basin, Central China 355

Figure 7. Interstitial material contents of P2h8, P1s1, P1s2, and C2b members in the Yanchang gas field.

Figure 8. Relation between porosity and permeability in P1s2 of the Upper Paleozoic. Points in full line ellipses exhibit high permeability but low porosity, and the points in the dotted line ellipse exhibit low permeability but high porosity. lithology was dominated by quartz sandstone. The particles meability of gas reservoirs is higher due to the presence of mi- were mainly medium particles and medium-coarse particles cro-cracks. In areas where inter-granular holes and corrosion are (0.25–1.0 mm). The average interstitial material content of the relatively strong, porosity and permeability are higher (Fig. 8). matrixes was 10.2% (5.2%–26.0%), mainly ferro-calcite, silica, The porosities in the four gas formation decrease along with and ferro-dolomite. increasing reservoir depth due to increasing compaction as buri-

Scanning electron microscope observations and rock and al depth increased. The permeability of P1s2 was significantly cast sheet statistics reveal that pore types in the P2h8–C2b in- higher than permeability in the other three reservoirs (Table 1), clude residual inter-granular, dissolved inter-granular, dissolved due to the relatively greater micro-crack development in P1s2 intra-granular, and dissolved intra-interstitial material, authi- (Fig. 8). genic intra-crystal minerals, and a small proportion of micro-fractured pores (Chart II). Secondary pores, such as in- 6 IMPLICATIONS FOR SIMILAR EXPLORATION ter-granular dissolved pores, intra-granular pores, and in- By the end of 2014, exploration for natural gas in the Or- ter-crystal pores, comprise the main reservoir space. dos Basin has been ongoing for approximately 60 years, and Correlation between the porosity and permeability of the exploration of Upper Paleozoic natural gas gradually extended

P2h8-C2b reservoirs is relatively good (Fig. 8). Even though the from the north to the southeast parts of the basin. Exploration differences between the maximum and minimum porosities and targets extended from river-delta plain sand bodies to delta permeability are relatively large, the porosities of the gas reser- shore front sand bodies. Following the discovery of the Yanc- voirs are mainly less than 10%. The permeability is generally hang gas field, exploration of the Ordos Basin progressed from less than 1 mD and show that the Yanchang gas field is a typical, a search for oil in the south and gas in the north to exploring a tight sandstone gas reservoir (Table 1). In some areas, the per- basin full of gas.

356 Hairen Gao and Weili Wang

Innovative ideas and methods have resulted in a surprising Slope. Petroleum Exploration and Development, 35(6): 664–667 (in increase of the production of natural gas in the basin. A Chinese with English Abstract) world-class gas field, the Sulige gas field, was discovered in Fu, J. H., Yao, J. L., Liu, X. S., 2010. Evaluation of the Natural Gas Rre- this way, and initiated the discovery of dominant structural and sources in the Palaeozoic of the Ordos Basin. Xi’an: PetroChina lithological traps while searching for high porosity and perme- Changqing Oilfield Company (in Chinese) able regions in tight gas reservoirs. From an initial stage of Guo, J., Chen, H. D., Wang, F., et al., 2012. Main Controlling Factors of searching for favorable regions of reservoir development and Taiyuan Formation Sand Body Distribution in Ordos Basin. investigating primary targeted reservoirs solely relying on out- Fault-Block Oil and Gas Field, 19(5): 568–571 (in Chinese with Eng- crop observations and drilling, to the current stage of inferring lish Abstract) sedimentary environment distribution relying on Guo, Y. H., Liu, H. J., 1999. Transgression of Late Paleozoic Era in Ordos high-resolution seismic technology, we have greatly expanded Area. Journal of China Universtiy of Mining &Technology, 28(2): areas and exploration depths when exploring for gas reservoirs, 126–129 (in Chinese with English Abstract) and have greatly reduced exploration costs and risks. From the Huang, J. S., Zhen, C. B., Zhang, J., 2005. Origin of the Central Paleo Uplift initial search for natural gas production without gas reservoir in E’Eduosi Basin. Natural Gas Industry, 25(4): 23–26(in Chinese with transformation to current tight sandstone gas reservoir opera- English Abstract) tions relying on continuous fracking technology improvements, Jiang, Z. X., Xu, J., Chen, Z. Y., et al., 2011. Sedimentary Systems and reservoirs with lower physical properties can be exploited. Their Iinfluences on Gas Distribution in the Second Nember and Third Before the discovery of the Yanchang gas field, the search Member of the Permian Xiashihezi Formation in the Daniudi Gas Field, for natural gas in the Ordos Basin was confined to a shallow Ordos Basin, China. Energy Exploration and Exploitation, 29(1): area in the northern region of the basin and the southern region 59–75. doi:10.1260/0144-5987.29.1.59 was thought to contain unproductive sand bodies. The discov- Li, D. S., 2004. Return to Petroleum Geology of Ordos Basin. Petroleum ery of the Yanchang gas field has not only revealed huge poten- Exploration and Development, 31(6): 1–7 (in Chinese with English tial natural gas reserves in the Ordos Basin but also increased Abstract) the determination of petroleum geological workers to explore Liu, C. Y., Zhao, H. G., Gui, X. J., et al., 2006. Space-Time Coordinates of deep-water areas. Along with the progress of seismic technolo- the Evolution and Reformation and Mineralization Response in Ordos gies and improvement of sedimentary environment depiction, Basin. Acta Geologica Sinica, 80(5): 617–636 (in Chinese with English more and more large gas reservoirs should be discovered in the Abstract) southern region of the Ordos Basin in future. Peng, H. Y., Chen, H. D., Xiang, F., et al., 2006. Application of Trace Ele- ment Analysis on Sedimentary Environment Identification——An ACKNOWLEDGMENTS Example from the Permian Shanxi Formation in Eastern Ordos Basin. 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