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Division of “layer exploration unit” and high-efficiency exploration in mature exploration area
PETROLEUM EXPLORATION AND DEVELOPMENT Volume 45, Issue 3, June 2018 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2018, 45(3): 544–552.
RESEARCH PAPER
Division of “layer exploration unit” and high-efficiency exploration in mature exploration area SONG Mingshui1,*, WANG Yongshi1, LI Youqiang2 1. Sinopec Shengli Oilfield Company, Dongying 257000, China; 2. Exploration and Development Research Institute of Sinopec Shengli Oilfield Company, Dongying 257000, China
Abstract: To realize high-efficiency and sustainable exploration of the Jiyang depression at the stage of high exploration degree, a hydrocarbon accumulation-geological evaluation method is developed on the basis of current geologic knowledge and extent of fine exploration. The concept of “layer exploration unit” is proposed in the study, and it is defined as an exploration geological unit that has a relatively complete and unified tectonic system, sedimentary system and hydrocarbon migration & accumulation system in a tectonic layer or tectonic sublayer within a fault basin. Then, an approach to dividing and evaluating the “layer exploration unit” is developed. With this approach, the Jiyang depression is divided into 305 layer exploration units, thus helping realize precise and stereoscopic geological understanding and exploration deployment. Fine splitting of remaining resources and benefit evaluation of exploration targets are conducted by “layer exploration units”. As a result, 66 efficient “layer exploration units” in four major areas (i.e. Paleogene upper Es4Dongying Formation, Neogene Minghuazhen FormationGuantao Formation, Paleozoic buried-hill, and Paleogene Kongdian Formationlower Es4) are determined as the targets for obtaining more reserves and breakthroughs in the short and medium term. Key words: Bohai Bay Basin; Jiyang depression; fault basin; mature exploration area; layer exploration unit; fine exploration; high-efficiency exploration
1. 1.1.
Exploration Overview Arising of the issue
Located in the southeast part of Bohai Bay Basin (Fig. 1), the Jiyang depression is a typical continental downfaulted basin in East China[1]. Since the exploration made breakthrough in 1961, the depression has experienced nearly 60 years of oil exploration and development. By the end of 2016, the Jiyang depression had an overall density of exploration well of 0.23 well/km2 and exploration degree of up to 51.9%. More than ten oil bearing formations have been discovered, upwardly Archean, Paleozoic, Mesozoic, and Cenozoic including Kongdian Formation (Ek) – lower part of the 4th member of Shahejie Formation (Es4L for short), the upper part of the 4th member of Shahejie Formation (Es4U for short), the 3rd member of Shahejie Formation (Es3 for short), the 2nd member of Shahejie Formation (Es2 for short), the 1st member of Shahejie Formation (Es1 for short), Dongying Formation (Ed), and Guantao Formation–Minghuazhen Formation (Ng–Nm). Among them, Es4U and layers shallower than Es4U are main contributors to reserves, and exploration degree of them has exceeded 60%. According to exploration degree
classification criterion recommended by SINOPEC, this area has reached high-ultra high exploration degree[23]. Since 2014, international oil prices fell in cliff style, and long fluctuated around low price. As a result, upstream enterprises of oil industry experienced an unprecedented serious
Fig. 1.
Location of Jiyang depression in Bohai Bay Basin.
Received date: 26 Jun. 2017; Revised date: 19 Apr. 2018. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the China National Science and Technology Major Project (2016ZX05006-003). Copyright © 2018, Research Institute of Petroleum Exploration & Development, PetroChina. Publishing Services provided by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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situation. Under this circumstance, the exploration of oil and gas should be focused on the economic feasibility and recoverability of newly discovered oil geologic reserves, so we must turn from looking for geologic reserves to looking for recoverable reserves. Subsequently, exploration in Jiyang depression has stepped into a new stage that emphasizes on benefit and efficiency in exploration, and thus refined exploration has become inevitable in the mature exploration area in downfaulted basin. Where are the remaining profitable reserves? How to select and evaluate efficient exploration areas? These are some of the questions waiting to be answered urgently[35]. 1.2.
Exploration history in Jiyang depression
Exploration belt is the basic focus point for exploration practice, and the scale of exploration belt is a comprehensive reflection of geological understanding and exploration degree. Scientifically dividing and selecting exploration belt is one of the main missions in every exploration stage. According to exploration practice in Jiyang depression, exploration belt is ever changing with variation of geologic understanding and exploration degree. Exploration history in Jiyang depression can be divided into 4 stages: the 1st stage (1960-1982), when the exploration was carried out in Dongying sag, Zhanhua sag, Chezhen sag, and Huimin sag to search for anticlinal structural reservoirs by using 2D seismic data and related techniques; the 2nd stage (1983-1995), guided by theory of composite oil-gas accumulation zone, exploration in this stage focused on looking for composite oil-gas accumulation zones in 26 secondary structural belts by using 3D seismic data and related techniques; the 3rd stage (1996-2012), guided by exploration theory on subtle trap, the exploration in this stage aimed at finding subtle reservoirs like lithologic, stratigraphic, complex faulted block, and buried hill reservoirs in 26 secondary structural belts by using high precision 3D seismic data and related supporting techniques; and the 4th stage (since 2013), guided by theory on orderliness and difference of reservoir distribution in oil-rich sag in downfaulted basin, refined exploration in search of complex subtle traps were carried out focusing on layer exploration unit by making use of prestack time migration or prestack depth migration 3D seismic data and related supporting techniques. From 1983 to 2012, proved original oil in place increased at 108 t a year in Jiyang depression for 30 years in a row, setting an example of exploration in continental downfaulted basin. Since 2013, under the dual pressure of high exploration degree and low oil price, controlled original oil in place has been increasing at 5 000×104 t a year, and all the proved reserves are high quality recoverable reserves. Scale of exploration unit adaptive to development stage of exploration is the precondition for researchers to understand geologic features of targets, evaluate resource potential, and carry out scientific deployment, and realize efficient exploration. In the mature exploration stage characterized by refined exploration and benefit priority in Jiyang depression, it is im-
portant to precisely evaluate remain resource potential, sort out areas or layers with relatively low exploration degree, and solve key problems restricting exploration. Exploration must focus on more precise unit and specific targets such as layers.
2. Concept and division method of layer exploration unit 2.1.
Concept of layer exploration unit
Exploration practice has proved that Jiyang depression, as a continental downfaulted basin, is a composite oil-gas accumulation zone characterized by rich faults, multiple types of traps, complex distribution of reservoir bodies, multiple stages of basin evolution, multiple sets of major source rocks, multistages of petroleum accumulation, and various reservoir types[1]. Such characteristics make reservoir forming patterns differ widely both on the plane and between different layer series. As result, different areas and different layers may have differences in petroleum enrichment degree, and exploration orientation should also be adjusted accordingly. To grasp the difference is the crux of exploration, and variation in exploration unit in different exploration stages is concrete embodiment of development in geologic understanding and deployment idea[68]. Although the depression is high in overall exploration degree, different layer series have quite big differences in exploration degree, understanding degree, and degree of discovery. This unevenness in exploration degree is determined by complexity of geologic features in the downfaulted basin and the law of subjective cognition. Layer exploration unit is defined as a geologic element for exploration with relatively unified structural system, sedimentary system and hydrocarbon migration and accumulation system within the same tectonic layer or sub-tectonic layer of a downfaulted basin. It is a basic geologic unit with relatively complete geologic features, reservoir forming features, mature geologic understanding and adaptable techniques, and is also a exploration unit with unified exploration idea and deployment strategy. By narrowing down the exploration to a layer series, layer exploration unit gives more specific areas and directions for the layer in concern. 2.2.
Division method of layer exploration unit
Division of layer exploration units must take into account factors such as reservoir forming conditions, exploration degree, exploration theory and techniques, working guideline, and exploration strategy, etc. Vertically, target layers need to be subdivided, and laterally, exploration blocks also need to be subdivided. Exploration is a continuous process, with exploration theories and techniques inheriting and developing at the same time. In principle, division of layer exploration unit should be carried out on basis of exploration blocks. 2.2.1.
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Laterally subdivision of exploration blocks
Lateral division scale of exploration unit determines the un-
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derstanding precision of a working area. Exploration unit is a relatively independent geologic element with complete structural system, sedimentary system and petroleum accumulation system, and is also a basic working unit under unified study and deployment. Division of exploration units should take into full consideration of structural system, sedimentary system, migration and accumulation system as well as difference in exploration degree. Taking the Chengdao area as an example, it is a big scaled independent drape structural belt above buried hill (Fig. 2). Bounded by Chengbei fault in the west, Prospecting Right boundary in the north and east, and Chengbei-15fault in the south from Zhuangxi subsag, it is a typical drape anticlinal structural belt. Chengdao area possess favorable oil source conditions with Chengbei sag to the west, Bozhong sag to the east, Sha'nan subsag to the north, and Zhuangxi subsag to the south. Chengdao area has been explored as a composite oil-gas accumulation zone in the past. Based on structural system, petroleum accumulation system, sedimentary system, and exploration degree, Chengdao area is divided into 3 exploration units: Chengdao low salient, south Chengdao fault belt, and east Chengdao slope. Among them, the Chengdao low salient is the main body of the drape anticlinal structure with simple structural pattern, where the Tertiary system wholly drapes on top of Pre-Tertiary. Surrounded by oil source areas, it is the structural belt richest in oil and gas in Chengdao area. In this structural belt, Neogene channel sand bodies have been main exploration targets, with higher exploration degree; while Eogene, Mesozoic, Paleozoic, and Archean, with some discoveries, are lower in exploration degree
Fig. 2.
Division of exploration units in Chengdao area.
than Neogene. The east Chengdao slope is also simpler in structural pattern, but there the exploration targets stratigraphic reservoirs and lithologic reservoirs in Paleogene, and the exploration degree is low due to large burial depth. The south Chengdao fault belt is complex in structural pattern, with many fault blocks and abundant petroleum resources. The exploration targets include channel sand bodies in Neogene, clastic rock buried hill in Mesozoic, and carbonate buried hill in Paleozoic. Generally, the exploration units are different in structural system, sedimentary system of main targets, and reservoir forming conditions. Small scaled subsags and salients with independent reservoir forming system and low exploration degree such as Lizezhen subsag belt, Liuzhong subsag belt, Qingtuozi salient, etc., can be considered as individual units for regional geologic study and exploration deployment. According to these division criterion, Jiyang depression is divided into 72 exploration units (Table 1). 2.2.2.
Vertically subdivision of target layers
Different layers have differences in exploration degree, structural system, sedimentary system, reservoir forming system, exploration techniques, and prospecting strategies. Altogether 12 oil-bearing layers have been discovered in Jiyang depression, and 9 of them are main target layers namely Ng– Nm during depression period, Es4L–Ek during initial downfaulted period, Es4U, Es3, and Es2 during intensive downfaulted period, Ed–Es1 during faded downfaulted period, Mesozoic and Paleozoic during rift period, and Archean the basement. According to exploration degree and petroleum discoveries, the target layers are divided into 3 categories, and first is mature exploration layer including Ng–Nm, Es4U, Es3, and Es2 in Eogene; the second category is highly encountered low understanding layer including Ed-Es1; and the third category is low exploration degree layer including Es3L–Ek in Eogene, Mesozoic, Paleozoic, and Archean. Ng–Nm is of high exploration degree; dominated by fluvial system, shallow in burial depth and good in reservoir properties, it is a profitable exploration target, where the reservoir forming pattern is mainly meshwork-carpet style beside pressure compartment. Es4U is the main source rock layer, dominated by nearshore fan and lacustrine sediments such as steep slope nearshore subaqueous fans and gentle slope beach bar sediments; reservoir forming patterns are mainly glutenite reservoir sealed by upper fan beside pressure compartment and three-element controlled beach-bar reservoir beside pressure compartment. Es3 was deposited in the peak period of downfaulted tectonic movement, so there developed various sedimentary systems, including long-axis delta-turbidity current and short-axis nearshore subaqueous fans. Main reservoir forming patterns include turbidite subtle reservoir within pressure compartment and glutenite reservoir sealed by upper fan beside pressure compartment. Es2 is dominated by delta plain and fluvial facies, where reservoirs are mainly controlled by structural fea-
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tures. Es1-Ed depositing in the transition period between depression and down-fault, is composed of lacustrine carbonate rock and distal delta facies principally. Es1–Ed is far from matured source rock, where reservoir bed and trap size are main factors controlling petroleum enrichment and reservoir scale. Although frequently encountered by drilling, it is not well understood. Es4L–Ek was deposited in saline environment, with sedimentary facies similar to downfaulting period. But it forms an independent petroleum system due to a thick salt-gypsum bed at the top. Mesozoic is mainly composed of continental clastic sediments, with locally developed volcanic rock. Lower Paleozoic mainly consists of marine sediments and upper Paleozoic is mainly of transitional facies. Archean is mainly of gneiss buried hill. According to present understanding, reservoirs in Mesozoic, Paleozoic and Archean are mainly made up of new source rock layers and old reservoirs, and they are low in exploration degree. Differences in structural belt, reservoir bed and reservoir forming models between different target layers make it necessary to take different deployment strategy and exploration techniques for different target layers, and this is also the direct reason causing difference in exploration degree in different layers. Strata distribution varies in different structural belt, for example, in Chengdao area there only developed Ng–Nm, Ed, Es1, Mesozoic, Paleozoic, and Archean (Fig. 3), therefore, target layers for exploration must be determined according to actual situation of the exploration unit. 2.2.3.
Division of layer exploration units
Layer exploration unit is determined by appropriate matching of target layers with exploration units. Take Chengdao area as an example, it is divided into 9 layer exploration units according to target layers in different exploration units, and they are: ① Neogene in Chengdao low salient; ② Eogene in Chengdao low salient (mainly Es1 –Ed); ③ Mesozoic in Chengdao low salient; ④ Paleozoic in Chengdao low salient; ⑤ Archean in Chengdao low salient;
Fig. 3.
Reservoir-forming patterns in Chengdao area.
⑥ Eogene in east Chengdao slope (Es1-Ed); ⑦ Neogene in south Chengdao fault belt; ⑧ Mesozoic in south Chengdao fault belt; and ⑨ Paleozoic in south Chengdao fault belt (Fig. 4). According to this method, 72 exploration units in Jiyang depression are preliminarily divided into 305 layer exploration units (Table 1, Fig. 5). Based on division of layer exploration units, it is possible to assess remaining resources and select exploration direction, which can provide reference for refined exploration and management decision. Due to complexity of the continental downfaulted basin, different oil bearing layers differ widely in lateral distribution. Statistics show that no block in Jiyang depression has all the oil-bearing layers. Therefore, the number of layer exploration units is not simply the product of number of oil bearing layers and that of exploration blocks. Resonable division of layer exploration units need to be divided according to exploration practice. What needs to be pointed out is division of layer exploration units is dynamically changing in accordance with exploration practice. For composite petroleum accumulation areas contributing most reserve increment with high exploration degree, and multiple target layers with different exploration degrees and different reservoir forming conditions, it is recommended to subdivide into every formation even to member and sub-member. For example, Shehejie Formation in Jiyang depression is divided into four members upwardly Es4, Es3, Es2, and Es1, and Es4 is further divided into Es4L and Es4U. As for areas with low exploration degree or one target layer, or small difference between target layers, rough division of layer exploration unit would do. Lizezhen subsag is such an example, only Es4U layer exploration unit is identified there according to present understandings due to low exploration degree. Layer exploration unit is bigger than development block, but is more refined than exploration belt and exploration block, and it is specific target for research and management in mature areas.
3. Assessment on remaining resources in layer exploration unit The exploration value of a layer exploration unit is primarily dependent on its remaining resources and economic profit, and secondarily on current exploration techniques and surface conditions. Exploration block is the basement for layer exploration unit, therefore, the assessment of remaining resources of a layer exploration unit should also be based on resources assessment of the block. Before new results of resource assessment comes, resources of a layer exploration unit should be worked out according to available resource assessment result by means of forward modeling and backward modeling. The following basic principles are recommended: (1) Resources of a layer exploration unit must bigger than all discovered reserves in it. (2) Resources of oil discovery points outside of the proved area, resources in reserved traps, and possible resources in
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Table 1. Division of exploration blocks in different exploration stages in Jiyang depression. Number of Number of blocks Number of blocks at present blocks at struc- at stage of complex Explo- Layer ration explounit ration unit Dongying sag 1 4 20 93 Zhanhua sag 1 8 20 103 Chezhen sag 1 2 9 39 Huimin sag 1 7 11 39 Tanhai area 1 5 12 31 Total 5 26 72 305 Sag
Fig. 5. Histogram of layer exploration units in different sags of Jiyang depression.
areas or layers not assessed should be taken into account. (3) Existing reservoir forming rules and mid-long term target should be taken into consideration, for example, main layer exploration unit for reserve increasing, secondary layer exploration unit for reserve increasing, and possible layer exploration unit for reserve increasing should be dealt with in different ways. (4) Exploratory well density and degree of resource discovery of the layer exploration unit should be considered too. Therefore, resources of a layer exploration unit = proved reserves of the unit + controlled reserves of the unit + predicted reserves of the unit + resources in oil bearing traps + resources in reserved traps + possible resources in non-assessed area. The remaining resources of a layer exploration unit are the resources of the unit minus proved reserves and controlled reserves. According to the principles and methods above, resources of 305 layer exploration units in Jiyang depression were calculated to determine the distribution of the remaining resources in Jiyang depression. Overall, 147 layer exploration units have the remaining resources of more than 1 000×104 t, and 19 layer exploration units have the remaining resource abundance of more than 20×104 t/km2. Apparently, there are still considerable remaining resources and higher resource abundance, but different layers are different somewhat.
4. Fig. 4.
Division of layer exploration units in Chengdao area.
Selection of effective layer exploration units The precondition of efficient exploration is to select effi-
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cient layer exploration units accurately. The efficient means the resources are highly profitable, recoverable under present theory and techniques, and surface condition is ready for the development of the resources (this is important in ever growing East China). Among them, economic benefit is precondition, techniques are key factors, and surface permit is the basement. Detailed procedures for the evaluation of remaining resources are as follows: The 1st step: assessment of economic risk. Layer exploration units with different exploration degrees differ in exploration objectives and should be dealt with in different ways. For units of reserve increasing, the evaluation should focus on resource quality and successful rate of commercial discoveries. Balance oil price and commercial oil flow rate are two indexes to assess the risk. Units for breakthrough should focus on resource discovery, with exploration potential and leading significance paid attention to. Resource scale and industrial oil flow standard are major factors to be considered in the evaluation. According to assessment results, 103 profitable ones are selected out of 305 layer exploration uints in Jiyang depression. The 2nd step: assessment of theoretical and technical risk. Since the 11th Five Year Plan, exploration practice shows key factors causing failure of exploratory wells including wrong interpretations of reservoir, trap, migration, and position of structure, etc. The wrong interpretations are related to seismic data quality, misunderstandings in rules of reservoir bed development and petroleum accumulation, and adaptability of geophysical techniques. Clearly, if the geological understanding of the layer exploration unit is accurate or the exploration technique is adaptable is directly related to the reliability of exploration target and successful rate of drilling. When it comes to the reliability of geologic understanding, the source rock, sedimentary system, development rules of effective reservoir bed, reservoir type and accumulation models should be considered. When considering the adaptability of geophysical techniques, target identification ability of seismic data, validity of the geophysical technique, recognition precision, successful rate of the technique should be taken into account. Reliability of geologic understanding and adaptability of geophysical techniques are quantitatively evaluated in Fig. 6. Preliminary evaluation shows among the 103 profitable layer exploration units, 92 units are theoretically and technically feasible. The 3rd step: assessment of surface conditions. Continental downfaulted basins in East China are mainly located in economically developed area. Economy and society grow fast at the same time of petroleum exploration, as cities and towns get denser, the surface space left for oil exploration is more and more limited. Furthermore, strengthened protection to natural environment also restricts space for exploration and development. Jiyang depression faces the sea to the east, and is mainly located in Dongying City and Binzhou City on land, which are both highly developed cities. Surface restricted
Fig. 6.
Evaluation template for reserve increasing units.
areas mainly include natural protection area of the Yellow River Delta, functional sea areas, urban development zones, aquaculture area, airport, and urban functional zones such as stations, etc. Among the 92 feasible layer exploration units, 66 units have favorable ground conditions (Table 2). To sum up, the 66 layer exploration units are the most important targets for efficient exploration in Jiyang depression.
5. Efficient exploration domain during the 13th Five Year Plan The selected 66 layer exploration units are divided into 4 categories according to layer of interest, and they are separately Ng–Nm, Es4U–Ed, Paleozoic buried hill, and Es4L-Ek. According to exploration degree and understanding degree, they are divided into two categories, namely units for reserve increasing and units for exploration breakthrough. 5.1.
Domain for recent reserve increasing
Es4UEd and NgNm are both high in exploration degree and understanding degree, and have always been major layers for reserve increasing in Jiyang depression. Es4UEd deposited during main downfaulted period is the major source rock and the most important layer for reserve increasing in Jiyang depression. Despite of high exploration degree, there still exists a considerable blank area of 4 100 km2 between reserve areas, accounting for almost half of the area within the reserve enveloping lines. There are many oil flow wells in the blank area, where potential reserves may exist. Exploration practice has proved that during major downfaulted period, sedimentary system, migration system and fluid pressure developed in a continuous and orderly manner, which resulted in the orderly distributed reservoir types namely stratigraphic reservoir, structural reservoir, structural-lithologic reservoir, and lithologic reservoir from subsag area toward slope belt. Such orderliness also exists in every secondary sequence, and this orderliness is effective for prediction of reservoir types in blank exploration areas[912]. Blank areas without reserve are mainly located in transitional
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Table 2.
Evaluation results of layer exploration units in Jiyang depression.
Increasing reserves
Es4U-Ed (Structural zone with complex sedimentation) Es4U-Ed (abrupt change in strata distribution)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
Es4U-Ed (Tectonic transition zone )
No. Domain
35 36 37 38 39 40 41
55 56 57 58 59 60 61 62 63 64 65 66
Kongdian Formation-Es4L
54
Breakthrough
43 44 45 46 47 48 49 50 51 52 53
Paleozoic (Buried hill)
Ng-Nm
42
Layer exploration unit
Reservoir type
Es4U in Yanjia-Yong'anzhen Es4U in Lijin fault belt Es4U in Shengtuo fault belt Es3 in Yanjia-Yong'anzhen Es3 in the north part of Chexi Es3 in the north part of Bonan Es3 around Binxian bulge Es3 in the north part of Dawangbei Es3 in the north part of Taoerhe Es4U in Kenxi Es3 in Kenxi Es4U in Dawangzhuang U Es3 in Jiangjiadian-Wawu Es3 in Xiaozhuang Es4U in Chenguanzhuang-Wangjiagang Es4U in Shanghe U Es4 in Yihezhuang bulge Es2 in Chenguanzhuang-Wangjiagang Es2 in Shanghe Es2 in Shengtuo Es2 in Dawangzhuang Es4U in Shaojia-Sikou Es3 in Bonan Es3 in Niuzhuang Es3 in Jiangjiadian-Wawu Es3 in Minfeng subsag Es2 in Bonan subsag Es2 in Boxing subsag Es1 in Boxing subsag Es2 in Guangli Es1 in Dawangzhuang Es2 around Binxian bulge Es2 in Kenxi Es3-Guantao Formation around Qingtuozi Es3-Guantao Formation in the east part of the north slope in Chenjiazhuang Shahejie Formation in Qianguantun Es3-Es1 in Gaoqing area Es2-Es1 around Yihezhuang bulge Es2-Es3 in the south slope of Gubei Es3 in Wangjiagang Es3 in Liuqiao-Caoqiao Guantao Formation in the east and west edge of Chengdao area Guantao Formation in Zhuanghai area Guantao Formation in north part of Kendong Guantao Formation in east part of Kendong Guantao Formation in Sanhecun Upper Paleozoic in Qingcheng bulge Upper Paleozoic in Chenguanzhuang-Wangjiagang Lower Paleozoic in Shangdian-Pingfangwang Lower Paleozoic in Taoerhe Lower Paleozoic in Dawangzhuang Paleozoic in the north part of Chexi Lower Paleozoic in Yibei Lower Paleozoic in east part of the north slope in Chenjiazhuang bulge Lower Paleozoic in Chengdao buried hill Lower Paleozoic in Kendong buried hill Lower Paleozoic in Changdi buried hill Es4L in Shangdian-Pingfangwang Es4L in Boxing subsag belt Kongdian Formation Es4L in Chenguanzhuang-Wangjiagang Es4L in Yanjia-Yong'anzhen Es4L around Yi176-Boshen4 Ek1 in Guangli-Qingnan Ek2 in central part of Dongying Es4L in Lijin Es4L in Dawangbei
Lithologic Lithologic Lithologic Structural-lithologic Lithologic Structural-lithologic Structural-lithologic Lithologic Lithologic Structural-lithologic Structural-lithologic Structural-lithologic Structural Structural Structural Structural Structural-lithologic Structural Structural Structural Structural Lithologic Structural-lithologic Lithologic Structural-lithologic Lithologic Lithologic Lithologic Lithologic Structural-lithologic Structural-lithologic Structural-lithologic Structural-lithologic Stratigraphic
Evaluation of Reserves increment during the last 3 theoretical years in the 13th Five Year Plan/104 t and technical Controlled Predicted reliability reserves reserves I 1 000 II1 500 II2 300 I 500 II1 500 I 300 I 300 II1 300 II1 300 II2 500 II2 500 I 300 I 300 II1 400 I 400 II1 300 I 300 I 300 I 200 I 100 I 500 II2 600 I 500 II1 500 I 300 I 300 II1 500 II1 400 II2 300 II1 300 II1 300 II1 200 II1 200 II1 700 2 000
Stratigraphic
II1
600
1 500
Stratigraphic Stratigraphic Stratigraphic Stratigraphic Stratigraphic Stratigraphic
II1 II1 I II1 I II2
500 300 300 200 200 200
1 000 500 500 500 400 400
Structural, lithologic
I
1 500
2 000
Structural Structural Structural Structural, lithologic Buried hill Buried hill Buried hill Buried hill Buried hill Buried hill Buried hill
I I I II2 II2 II1 II1 II1 II1 II1 II1
1 500 2 000 1 000 400 500 400 500 200 300 1 000 300
1 500 1 000 1 000 500 500 2 500 500
Buried hill
II1
300
500
Buried hill Buried hill Buried hill Structural with other source Structural with other source
II1 II1 II1 I I
2 000
1 000 500 500
Authigenic codensate oil
II1
1 000
Authigenic codensate oil Authigenic codensate oil Authigenic codensate oil Authigenic codensate oil Authigenic codensate oil Structural with other source
II1 II1 II1 II1 II1 II1
1 000 800 500
550
300 200
3 000
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zone between structural belts, complex sedimentary areas, and areas with abrupt change in strata, correspondingly, there separately develop complex fault block reservoirs, complex lithologic reservoirs, and stratigraphic reservoirs. Among the three kinds of blank areas, assessment results reveal 41 effective layer exploration units may contribute reserves of more than 2.1×108 t during the 13th Five Year Plan. Ng–Nm depositing during depression period consists of primarily channel sands. With shallow reservoir depth and good reservoir quality, this layer is an efficient reserve increasing layer[13]. This layer is high in exploration degree in the main area of the big scale drape structural belt, therefore the flank areas of drape structures are the major exploration targets at present. As in these areas, the sand bodies are usually thin and low in oil fullness, so more detailed work need to be done including sand body description, hydrocarbon detection, etc. According to assessment, 5 layer exploration, such as Chengdao area, Zhuanghai area, etc, are expected to contribute reserves of more than 1.1×108 t. 5.2.
Domain for exploration breakthrough
Paleozoic buried hill and Es4LEk, both of low exploration degree and understanding degree, are regarded as areas for exploration breakthrough in Jiyang depression. The Paleozoic buried hill layer is a layer rich in oil in Jiyang depression. The buried hills in Jiyang depression are controlled by northwest striking faults into belts, and controlled by northeast striking faults in relief, and different types of buried hills are distributed in certain order from the south gentle slope to the north abrupt slope, where burial depth almost has no influence on reservoir properties. The main source rock in Es3L and Es4U of Eogene and coal measure source rock in Carboniferous and Permian provide sufficient petroleum resources[14]. Bottom depth of petroleum charging is the low limit for reservoir forming, and buried hills above the source injection window are all favorable traps. By combining distribution of source rock with distribution of buried hills, 11 efficient layer exploration units have been confirmed with optimal selection method, which are predicted to contribute reserves of more than 1.5×108 t. Es4LEk were deposited during initial downfaulted period in Jiyang depression. Under arid climate and shallow water body conditions, there developed 3 series of widely distributed thick stable salt-gypsum beds. Under the good sealing of the salt-gypsum beds, Es4LEk forms an independent petroleum system. Saline environment promoted efficient hydrocarbon generation and expulsion[1422], so self-generated reservoir is the important direction for current and mid-term exploration of this layer. With self-generated condensate oil reservoirs as the major targets, and structural reservoirs sourced by other source rock as secondary targets, 9 layer exploration units have been selected as favorable targets, namely Ek in GuangliQingnan, Es4L in YanjiaYong'anzhen, etc.
6.
during stage of high exploration degree can sort out efficient orientation and drilling targets, and thus decrease exploration risks and increase profit in downfaulted basins. Layer exploration unit is the basic element for exploration practice during the stage of high exploration degree, and is also basic geologic unit with relatively complete structure, sedimentation, and accumulation system, which reflects the consistency between objective geologic conditions and subjective cognition. The concept of layer exploration unit provided new idea for exploration in mature area. Basic contents of layer exploration unit evaluation include reasonable division of remaining resources, economic evaluation, adaptability of present geologic model and exploration techniques, and evaluation of surface engineering technology. For the purpose of efficient exploration, 305 layer exploration units have been identified in Jiyang depression , among which 66 are deemed to be profitable, geologically reliable, technically feasible under current economic situation, and have surface condition suitable for exploration, and results of this study have provided good reference for the decisionmaking on exploration in the 13th Five Year Plan.
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RESEARCH PAPER
Division of “layer exploration unit” and high-efficiency exploration in mature exploration area SONG Mingshui1,*, WANG Yongshi1, LI Youqiang2 1. Sinopec Shengli Oilfield Company, Dongying 257000, China; 2. Exploration and Development Research Institute of Sinopec Shengli Oilfield Company, Dongying 257000, China
Abstract: To realize high-efficiency and sustainable exploration of the Jiyang depression at the stage of high exploration degree, a hydrocarbon accumulation-geological evaluation method is developed on the basis of current geologic knowledge and extent of fine exploration. The concept of “layer exploration unit” is proposed in the study, and it is defined as an exploration geological unit that has a relatively complete and unified tectonic system, sedimentary system and hydrocarbon migration & accumulation system in a tectonic layer or tectonic sublayer within a fault basin. Then, an approach to dividing and evaluating the “layer exploration unit” is developed. With this approach, the Jiyang depression is divided into 305 layer exploration units, thus helping realize precise and stereoscopic geological understanding and exploration deployment. Fine splitting of remaining resources and benefit evaluation of exploration targets are conducted by “layer exploration units”. As a result, 66 efficient “layer exploration units” in four major areas (i.e. Paleogene upper Es4Dongying Formation, Neogene Minghuazhen FormationGuantao Formation, Paleozoic buried-hill, and Paleogene Kongdian Formationlower Es4) are determined as the targets for obtaining more reserves and breakthroughs in the short and medium term. Key words: Bohai Bay Basin; Jiyang depression; fault basin; mature exploration area; layer exploration unit; fine exploration; high-efficiency exploration
1. 1.1.
Exploration Overview Arising of the issue
Located in the southeast part of Bohai Bay Basin (Fig. 1), the Jiyang depression is a typical continental downfaulted basin in East China[1]. Since the exploration made breakthrough in 1961, the depression has experienced nearly 60 years of oil exploration and development. By the end of 2016, the Jiyang depression had an overall density of exploration well of 0.23 well/km2 and exploration degree of up to 51.9%. More than ten oil bearing formations have been discovered, upwardly Archean, Paleozoic, Mesozoic, and Cenozoic including Kongdian Formation (Ek) – lower part of the 4th member of Shahejie Formation (Es4L for short), the upper part of the 4th member of Shahejie Formation (Es4U for short), the 3rd member of Shahejie Formation (Es3 for short), the 2nd member of Shahejie Formation (Es2 for short), the 1st member of Shahejie Formation (Es1 for short), Dongying Formation (Ed), and Guantao Formation–Minghuazhen Formation (Ng–Nm). Among them, Es4U and layers shallower than Es4U are main contributors to reserves, and exploration degree of them has exceeded 60%. According to exploration degree
classification criterion recommended by SINOPEC, this area has reached high-ultra high exploration degree[23]. Since 2014, international oil prices fell in cliff style, and long fluctuated around low price. As a result, upstream enterprises of oil industry experienced an unprecedented serious
Fig. 1.
Location of Jiyang depression in Bohai Bay Basin.
Received date: 26 Jun. 2017; Revised date: 19 Apr. 2018. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the China National Science and Technology Major Project (2016ZX05006-003). Copyright © 2018, Research Institute of Petroleum Exploration & Development, PetroChina. Publishing Services provided by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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situation. Under this circumstance, the exploration of oil and gas should be focused on the economic feasibility and recoverability of newly discovered oil geologic reserves, so we must turn from looking for geologic reserves to looking for recoverable reserves. Subsequently, exploration in Jiyang depression has stepped into a new stage that emphasizes on benefit and efficiency in exploration, and thus refined exploration has become inevitable in the mature exploration area in downfaulted basin. Where are the remaining profitable reserves? How to select and evaluate efficient exploration areas? These are some of the questions waiting to be answered urgently[35]. 1.2.
Exploration history in Jiyang depression
Exploration belt is the basic focus point for exploration practice, and the scale of exploration belt is a comprehensive reflection of geological understanding and exploration degree. Scientifically dividing and selecting exploration belt is one of the main missions in every exploration stage. According to exploration practice in Jiyang depression, exploration belt is ever changing with variation of geologic understanding and exploration degree. Exploration history in Jiyang depression can be divided into 4 stages: the 1st stage (1960-1982), when the exploration was carried out in Dongying sag, Zhanhua sag, Chezhen sag, and Huimin sag to search for anticlinal structural reservoirs by using 2D seismic data and related techniques; the 2nd stage (1983-1995), guided by theory of composite oil-gas accumulation zone, exploration in this stage focused on looking for composite oil-gas accumulation zones in 26 secondary structural belts by using 3D seismic data and related techniques; the 3rd stage (1996-2012), guided by exploration theory on subtle trap, the exploration in this stage aimed at finding subtle reservoirs like lithologic, stratigraphic, complex faulted block, and buried hill reservoirs in 26 secondary structural belts by using high precision 3D seismic data and related supporting techniques; and the 4th stage (since 2013), guided by theory on orderliness and difference of reservoir distribution in oil-rich sag in downfaulted basin, refined exploration in search of complex subtle traps were carried out focusing on layer exploration unit by making use of prestack time migration or prestack depth migration 3D seismic data and related supporting techniques. From 1983 to 2012, proved original oil in place increased at 108 t a year in Jiyang depression for 30 years in a row, setting an example of exploration in continental downfaulted basin. Since 2013, under the dual pressure of high exploration degree and low oil price, controlled original oil in place has been increasing at 5 000×104 t a year, and all the proved reserves are high quality recoverable reserves. Scale of exploration unit adaptive to development stage of exploration is the precondition for researchers to understand geologic features of targets, evaluate resource potential, and carry out scientific deployment, and realize efficient exploration. In the mature exploration stage characterized by refined exploration and benefit priority in Jiyang depression, it is im-
portant to precisely evaluate remain resource potential, sort out areas or layers with relatively low exploration degree, and solve key problems restricting exploration. Exploration must focus on more precise unit and specific targets such as layers.
2. Concept and division method of layer exploration unit 2.1.
Concept of layer exploration unit
Exploration practice has proved that Jiyang depression, as a continental downfaulted basin, is a composite oil-gas accumulation zone characterized by rich faults, multiple types of traps, complex distribution of reservoir bodies, multiple stages of basin evolution, multiple sets of major source rocks, multistages of petroleum accumulation, and various reservoir types[1]. Such characteristics make reservoir forming patterns differ widely both on the plane and between different layer series. As result, different areas and different layers may have differences in petroleum enrichment degree, and exploration orientation should also be adjusted accordingly. To grasp the difference is the crux of exploration, and variation in exploration unit in different exploration stages is concrete embodiment of development in geologic understanding and deployment idea[68]. Although the depression is high in overall exploration degree, different layer series have quite big differences in exploration degree, understanding degree, and degree of discovery. This unevenness in exploration degree is determined by complexity of geologic features in the downfaulted basin and the law of subjective cognition. Layer exploration unit is defined as a geologic element for exploration with relatively unified structural system, sedimentary system and hydrocarbon migration and accumulation system within the same tectonic layer or sub-tectonic layer of a downfaulted basin. It is a basic geologic unit with relatively complete geologic features, reservoir forming features, mature geologic understanding and adaptable techniques, and is also a exploration unit with unified exploration idea and deployment strategy. By narrowing down the exploration to a layer series, layer exploration unit gives more specific areas and directions for the layer in concern. 2.2.
Division method of layer exploration unit
Division of layer exploration units must take into account factors such as reservoir forming conditions, exploration degree, exploration theory and techniques, working guideline, and exploration strategy, etc. Vertically, target layers need to be subdivided, and laterally, exploration blocks also need to be subdivided. Exploration is a continuous process, with exploration theories and techniques inheriting and developing at the same time. In principle, division of layer exploration unit should be carried out on basis of exploration blocks. 2.2.1.
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Laterally subdivision of exploration blocks
Lateral division scale of exploration unit determines the un-
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derstanding precision of a working area. Exploration unit is a relatively independent geologic element with complete structural system, sedimentary system and petroleum accumulation system, and is also a basic working unit under unified study and deployment. Division of exploration units should take into full consideration of structural system, sedimentary system, migration and accumulation system as well as difference in exploration degree. Taking the Chengdao area as an example, it is a big scaled independent drape structural belt above buried hill (Fig. 2). Bounded by Chengbei fault in the west, Prospecting Right boundary in the north and east, and Chengbei-15fault in the south from Zhuangxi subsag, it is a typical drape anticlinal structural belt. Chengdao area possess favorable oil source conditions with Chengbei sag to the west, Bozhong sag to the east, Sha'nan subsag to the north, and Zhuangxi subsag to the south. Chengdao area has been explored as a composite oil-gas accumulation zone in the past. Based on structural system, petroleum accumulation system, sedimentary system, and exploration degree, Chengdao area is divided into 3 exploration units: Chengdao low salient, south Chengdao fault belt, and east Chengdao slope. Among them, the Chengdao low salient is the main body of the drape anticlinal structure with simple structural pattern, where the Tertiary system wholly drapes on top of Pre-Tertiary. Surrounded by oil source areas, it is the structural belt richest in oil and gas in Chengdao area. In this structural belt, Neogene channel sand bodies have been main exploration targets, with higher exploration degree; while Eogene, Mesozoic, Paleozoic, and Archean, with some discoveries, are lower in exploration degree
Fig. 2.
Division of exploration units in Chengdao area.
than Neogene. The east Chengdao slope is also simpler in structural pattern, but there the exploration targets stratigraphic reservoirs and lithologic reservoirs in Paleogene, and the exploration degree is low due to large burial depth. The south Chengdao fault belt is complex in structural pattern, with many fault blocks and abundant petroleum resources. The exploration targets include channel sand bodies in Neogene, clastic rock buried hill in Mesozoic, and carbonate buried hill in Paleozoic. Generally, the exploration units are different in structural system, sedimentary system of main targets, and reservoir forming conditions. Small scaled subsags and salients with independent reservoir forming system and low exploration degree such as Lizezhen subsag belt, Liuzhong subsag belt, Qingtuozi salient, etc., can be considered as individual units for regional geologic study and exploration deployment. According to these division criterion, Jiyang depression is divided into 72 exploration units (Table 1). 2.2.2.
Vertically subdivision of target layers
Different layers have differences in exploration degree, structural system, sedimentary system, reservoir forming system, exploration techniques, and prospecting strategies. Altogether 12 oil-bearing layers have been discovered in Jiyang depression, and 9 of them are main target layers namely Ng– Nm during depression period, Es4L–Ek during initial downfaulted period, Es4U, Es3, and Es2 during intensive downfaulted period, Ed–Es1 during faded downfaulted period, Mesozoic and Paleozoic during rift period, and Archean the basement. According to exploration degree and petroleum discoveries, the target layers are divided into 3 categories, and first is mature exploration layer including Ng–Nm, Es4U, Es3, and Es2 in Eogene; the second category is highly encountered low understanding layer including Ed-Es1; and the third category is low exploration degree layer including Es3L–Ek in Eogene, Mesozoic, Paleozoic, and Archean. Ng–Nm is of high exploration degree; dominated by fluvial system, shallow in burial depth and good in reservoir properties, it is a profitable exploration target, where the reservoir forming pattern is mainly meshwork-carpet style beside pressure compartment. Es4U is the main source rock layer, dominated by nearshore fan and lacustrine sediments such as steep slope nearshore subaqueous fans and gentle slope beach bar sediments; reservoir forming patterns are mainly glutenite reservoir sealed by upper fan beside pressure compartment and three-element controlled beach-bar reservoir beside pressure compartment. Es3 was deposited in the peak period of downfaulted tectonic movement, so there developed various sedimentary systems, including long-axis delta-turbidity current and short-axis nearshore subaqueous fans. Main reservoir forming patterns include turbidite subtle reservoir within pressure compartment and glutenite reservoir sealed by upper fan beside pressure compartment. Es2 is dominated by delta plain and fluvial facies, where reservoirs are mainly controlled by structural fea-
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tures. Es1-Ed depositing in the transition period between depression and down-fault, is composed of lacustrine carbonate rock and distal delta facies principally. Es1–Ed is far from matured source rock, where reservoir bed and trap size are main factors controlling petroleum enrichment and reservoir scale. Although frequently encountered by drilling, it is not well understood. Es4L–Ek was deposited in saline environment, with sedimentary facies similar to downfaulting period. But it forms an independent petroleum system due to a thick salt-gypsum bed at the top. Mesozoic is mainly composed of continental clastic sediments, with locally developed volcanic rock. Lower Paleozoic mainly consists of marine sediments and upper Paleozoic is mainly of transitional facies. Archean is mainly of gneiss buried hill. According to present understanding, reservoirs in Mesozoic, Paleozoic and Archean are mainly made up of new source rock layers and old reservoirs, and they are low in exploration degree. Differences in structural belt, reservoir bed and reservoir forming models between different target layers make it necessary to take different deployment strategy and exploration techniques for different target layers, and this is also the direct reason causing difference in exploration degree in different layers. Strata distribution varies in different structural belt, for example, in Chengdao area there only developed Ng–Nm, Ed, Es1, Mesozoic, Paleozoic, and Archean (Fig. 3), therefore, target layers for exploration must be determined according to actual situation of the exploration unit. 2.2.3.
Division of layer exploration units
Layer exploration unit is determined by appropriate matching of target layers with exploration units. Take Chengdao area as an example, it is divided into 9 layer exploration units according to target layers in different exploration units, and they are: ① Neogene in Chengdao low salient; ② Eogene in Chengdao low salient (mainly Es1 –Ed); ③ Mesozoic in Chengdao low salient; ④ Paleozoic in Chengdao low salient; ⑤ Archean in Chengdao low salient;
Fig. 3.
Reservoir-forming patterns in Chengdao area.
⑥ Eogene in east Chengdao slope (Es1-Ed); ⑦ Neogene in south Chengdao fault belt; ⑧ Mesozoic in south Chengdao fault belt; and ⑨ Paleozoic in south Chengdao fault belt (Fig. 4). According to this method, 72 exploration units in Jiyang depression are preliminarily divided into 305 layer exploration units (Table 1, Fig. 5). Based on division of layer exploration units, it is possible to assess remaining resources and select exploration direction, which can provide reference for refined exploration and management decision. Due to complexity of the continental downfaulted basin, different oil bearing layers differ widely in lateral distribution. Statistics show that no block in Jiyang depression has all the oil-bearing layers. Therefore, the number of layer exploration units is not simply the product of number of oil bearing layers and that of exploration blocks. Resonable division of layer exploration units need to be divided according to exploration practice. What needs to be pointed out is division of layer exploration units is dynamically changing in accordance with exploration practice. For composite petroleum accumulation areas contributing most reserve increment with high exploration degree, and multiple target layers with different exploration degrees and different reservoir forming conditions, it is recommended to subdivide into every formation even to member and sub-member. For example, Shehejie Formation in Jiyang depression is divided into four members upwardly Es4, Es3, Es2, and Es1, and Es4 is further divided into Es4L and Es4U. As for areas with low exploration degree or one target layer, or small difference between target layers, rough division of layer exploration unit would do. Lizezhen subsag is such an example, only Es4U layer exploration unit is identified there according to present understandings due to low exploration degree. Layer exploration unit is bigger than development block, but is more refined than exploration belt and exploration block, and it is specific target for research and management in mature areas.
3. Assessment on remaining resources in layer exploration unit The exploration value of a layer exploration unit is primarily dependent on its remaining resources and economic profit, and secondarily on current exploration techniques and surface conditions. Exploration block is the basement for layer exploration unit, therefore, the assessment of remaining resources of a layer exploration unit should also be based on resources assessment of the block. Before new results of resource assessment comes, resources of a layer exploration unit should be worked out according to available resource assessment result by means of forward modeling and backward modeling. The following basic principles are recommended: (1) Resources of a layer exploration unit must bigger than all discovered reserves in it. (2) Resources of oil discovery points outside of the proved area, resources in reserved traps, and possible resources in
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Table 1. Division of exploration blocks in different exploration stages in Jiyang depression. Number of Number of blocks Number of blocks at present blocks at struc- at stage of complex Explo- Layer ration explounit ration unit Dongying sag 1 4 20 93 Zhanhua sag 1 8 20 103 Chezhen sag 1 2 9 39 Huimin sag 1 7 11 39 Tanhai area 1 5 12 31 Total 5 26 72 305 Sag
Fig. 5. Histogram of layer exploration units in different sags of Jiyang depression.
areas or layers not assessed should be taken into account. (3) Existing reservoir forming rules and mid-long term target should be taken into consideration, for example, main layer exploration unit for reserve increasing, secondary layer exploration unit for reserve increasing, and possible layer exploration unit for reserve increasing should be dealt with in different ways. (4) Exploratory well density and degree of resource discovery of the layer exploration unit should be considered too. Therefore, resources of a layer exploration unit = proved reserves of the unit + controlled reserves of the unit + predicted reserves of the unit + resources in oil bearing traps + resources in reserved traps + possible resources in non-assessed area. The remaining resources of a layer exploration unit are the resources of the unit minus proved reserves and controlled reserves. According to the principles and methods above, resources of 305 layer exploration units in Jiyang depression were calculated to determine the distribution of the remaining resources in Jiyang depression. Overall, 147 layer exploration units have the remaining resources of more than 1 000×104 t, and 19 layer exploration units have the remaining resource abundance of more than 20×104 t/km2. Apparently, there are still considerable remaining resources and higher resource abundance, but different layers are different somewhat.
4. Fig. 4.
Division of layer exploration units in Chengdao area.
Selection of effective layer exploration units The precondition of efficient exploration is to select effi-
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cient layer exploration units accurately. The efficient means the resources are highly profitable, recoverable under present theory and techniques, and surface condition is ready for the development of the resources (this is important in ever growing East China). Among them, economic benefit is precondition, techniques are key factors, and surface permit is the basement. Detailed procedures for the evaluation of remaining resources are as follows: The 1st step: assessment of economic risk. Layer exploration units with different exploration degrees differ in exploration objectives and should be dealt with in different ways. For units of reserve increasing, the evaluation should focus on resource quality and successful rate of commercial discoveries. Balance oil price and commercial oil flow rate are two indexes to assess the risk. Units for breakthrough should focus on resource discovery, with exploration potential and leading significance paid attention to. Resource scale and industrial oil flow standard are major factors to be considered in the evaluation. According to assessment results, 103 profitable ones are selected out of 305 layer exploration uints in Jiyang depression. The 2nd step: assessment of theoretical and technical risk. Since the 11th Five Year Plan, exploration practice shows key factors causing failure of exploratory wells including wrong interpretations of reservoir, trap, migration, and position of structure, etc. The wrong interpretations are related to seismic data quality, misunderstandings in rules of reservoir bed development and petroleum accumulation, and adaptability of geophysical techniques. Clearly, if the geological understanding of the layer exploration unit is accurate or the exploration technique is adaptable is directly related to the reliability of exploration target and successful rate of drilling. When it comes to the reliability of geologic understanding, the source rock, sedimentary system, development rules of effective reservoir bed, reservoir type and accumulation models should be considered. When considering the adaptability of geophysical techniques, target identification ability of seismic data, validity of the geophysical technique, recognition precision, successful rate of the technique should be taken into account. Reliability of geologic understanding and adaptability of geophysical techniques are quantitatively evaluated in Fig. 6. Preliminary evaluation shows among the 103 profitable layer exploration units, 92 units are theoretically and technically feasible. The 3rd step: assessment of surface conditions. Continental downfaulted basins in East China are mainly located in economically developed area. Economy and society grow fast at the same time of petroleum exploration, as cities and towns get denser, the surface space left for oil exploration is more and more limited. Furthermore, strengthened protection to natural environment also restricts space for exploration and development. Jiyang depression faces the sea to the east, and is mainly located in Dongying City and Binzhou City on land, which are both highly developed cities. Surface restricted
Fig. 6.
Evaluation template for reserve increasing units.
areas mainly include natural protection area of the Yellow River Delta, functional sea areas, urban development zones, aquaculture area, airport, and urban functional zones such as stations, etc. Among the 92 feasible layer exploration units, 66 units have favorable ground conditions (Table 2). To sum up, the 66 layer exploration units are the most important targets for efficient exploration in Jiyang depression.
5. Efficient exploration domain during the 13th Five Year Plan The selected 66 layer exploration units are divided into 4 categories according to layer of interest, and they are separately Ng–Nm, Es4U–Ed, Paleozoic buried hill, and Es4L-Ek. According to exploration degree and understanding degree, they are divided into two categories, namely units for reserve increasing and units for exploration breakthrough. 5.1.
Domain for recent reserve increasing
Es4UEd and NgNm are both high in exploration degree and understanding degree, and have always been major layers for reserve increasing in Jiyang depression. Es4UEd deposited during main downfaulted period is the major source rock and the most important layer for reserve increasing in Jiyang depression. Despite of high exploration degree, there still exists a considerable blank area of 4 100 km2 between reserve areas, accounting for almost half of the area within the reserve enveloping lines. There are many oil flow wells in the blank area, where potential reserves may exist. Exploration practice has proved that during major downfaulted period, sedimentary system, migration system and fluid pressure developed in a continuous and orderly manner, which resulted in the orderly distributed reservoir types namely stratigraphic reservoir, structural reservoir, structural-lithologic reservoir, and lithologic reservoir from subsag area toward slope belt. Such orderliness also exists in every secondary sequence, and this orderliness is effective for prediction of reservoir types in blank exploration areas[912]. Blank areas without reserve are mainly located in transitional
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Table 2.
Evaluation results of layer exploration units in Jiyang depression.
Increasing reserves
Es4U-Ed (Structural zone with complex sedimentation) Es4U-Ed (abrupt change in strata distribution)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
Es4U-Ed (Tectonic transition zone )
No. Domain
35 36 37 38 39 40 41
55 56 57 58 59 60 61 62 63 64 65 66
Kongdian Formation-Es4L
54
Breakthrough
43 44 45 46 47 48 49 50 51 52 53
Paleozoic (Buried hill)
Ng-Nm
42
Layer exploration unit
Reservoir type
Es4U in Yanjia-Yong'anzhen Es4U in Lijin fault belt Es4U in Shengtuo fault belt Es3 in Yanjia-Yong'anzhen Es3 in the north part of Chexi Es3 in the north part of Bonan Es3 around Binxian bulge Es3 in the north part of Dawangbei Es3 in the north part of Taoerhe Es4U in Kenxi Es3 in Kenxi Es4U in Dawangzhuang U Es3 in Jiangjiadian-Wawu Es3 in Xiaozhuang Es4U in Chenguanzhuang-Wangjiagang Es4U in Shanghe U Es4 in Yihezhuang bulge Es2 in Chenguanzhuang-Wangjiagang Es2 in Shanghe Es2 in Shengtuo Es2 in Dawangzhuang Es4U in Shaojia-Sikou Es3 in Bonan Es3 in Niuzhuang Es3 in Jiangjiadian-Wawu Es3 in Minfeng subsag Es2 in Bonan subsag Es2 in Boxing subsag Es1 in Boxing subsag Es2 in Guangli Es1 in Dawangzhuang Es2 around Binxian bulge Es2 in Kenxi Es3-Guantao Formation around Qingtuozi Es3-Guantao Formation in the east part of the north slope in Chenjiazhuang Shahejie Formation in Qianguantun Es3-Es1 in Gaoqing area Es2-Es1 around Yihezhuang bulge Es2-Es3 in the south slope of Gubei Es3 in Wangjiagang Es3 in Liuqiao-Caoqiao Guantao Formation in the east and west edge of Chengdao area Guantao Formation in Zhuanghai area Guantao Formation in north part of Kendong Guantao Formation in east part of Kendong Guantao Formation in Sanhecun Upper Paleozoic in Qingcheng bulge Upper Paleozoic in Chenguanzhuang-Wangjiagang Lower Paleozoic in Shangdian-Pingfangwang Lower Paleozoic in Taoerhe Lower Paleozoic in Dawangzhuang Paleozoic in the north part of Chexi Lower Paleozoic in Yibei Lower Paleozoic in east part of the north slope in Chenjiazhuang bulge Lower Paleozoic in Chengdao buried hill Lower Paleozoic in Kendong buried hill Lower Paleozoic in Changdi buried hill Es4L in Shangdian-Pingfangwang Es4L in Boxing subsag belt Kongdian Formation Es4L in Chenguanzhuang-Wangjiagang Es4L in Yanjia-Yong'anzhen Es4L around Yi176-Boshen4 Ek1 in Guangli-Qingnan Ek2 in central part of Dongying Es4L in Lijin Es4L in Dawangbei
Lithologic Lithologic Lithologic Structural-lithologic Lithologic Structural-lithologic Structural-lithologic Lithologic Lithologic Structural-lithologic Structural-lithologic Structural-lithologic Structural Structural Structural Structural Structural-lithologic Structural Structural Structural Structural Lithologic Structural-lithologic Lithologic Structural-lithologic Lithologic Lithologic Lithologic Lithologic Structural-lithologic Structural-lithologic Structural-lithologic Structural-lithologic Stratigraphic
Evaluation of Reserves increment during the last 3 theoretical years in the 13th Five Year Plan/104 t and technical Controlled Predicted reliability reserves reserves I 1 000 II1 500 II2 300 I 500 II1 500 I 300 I 300 II1 300 II1 300 II2 500 II2 500 I 300 I 300 II1 400 I 400 II1 300 I 300 I 300 I 200 I 100 I 500 II2 600 I 500 II1 500 I 300 I 300 II1 500 II1 400 II2 300 II1 300 II1 300 II1 200 II1 200 II1 700 2 000
Stratigraphic
II1
600
1 500
Stratigraphic Stratigraphic Stratigraphic Stratigraphic Stratigraphic Stratigraphic
II1 II1 I II1 I II2
500 300 300 200 200 200
1 000 500 500 500 400 400
Structural, lithologic
I
1 500
2 000
Structural Structural Structural Structural, lithologic Buried hill Buried hill Buried hill Buried hill Buried hill Buried hill Buried hill
I I I II2 II2 II1 II1 II1 II1 II1 II1
1 500 2 000 1 000 400 500 400 500 200 300 1 000 300
1 500 1 000 1 000 500 500 2 500 500
Buried hill
II1
300
500
Buried hill Buried hill Buried hill Structural with other source Structural with other source
II1 II1 II1 I I
2 000
1 000 500 500
Authigenic codensate oil
II1
1 000
Authigenic codensate oil Authigenic codensate oil Authigenic codensate oil Authigenic codensate oil Authigenic codensate oil Structural with other source
II1 II1 II1 II1 II1 II1
1 000 800 500
550
300 200
3 000
SONG Mingshui et al. / Petroleum Exploration and Development, 2018, 45(3): 544–552
zone between structural belts, complex sedimentary areas, and areas with abrupt change in strata, correspondingly, there separately develop complex fault block reservoirs, complex lithologic reservoirs, and stratigraphic reservoirs. Among the three kinds of blank areas, assessment results reveal 41 effective layer exploration units may contribute reserves of more than 2.1×108 t during the 13th Five Year Plan. Ng–Nm depositing during depression period consists of primarily channel sands. With shallow reservoir depth and good reservoir quality, this layer is an efficient reserve increasing layer[13]. This layer is high in exploration degree in the main area of the big scale drape structural belt, therefore the flank areas of drape structures are the major exploration targets at present. As in these areas, the sand bodies are usually thin and low in oil fullness, so more detailed work need to be done including sand body description, hydrocarbon detection, etc. According to assessment, 5 layer exploration, such as Chengdao area, Zhuanghai area, etc, are expected to contribute reserves of more than 1.1×108 t. 5.2.
Domain for exploration breakthrough
Paleozoic buried hill and Es4LEk, both of low exploration degree and understanding degree, are regarded as areas for exploration breakthrough in Jiyang depression. The Paleozoic buried hill layer is a layer rich in oil in Jiyang depression. The buried hills in Jiyang depression are controlled by northwest striking faults into belts, and controlled by northeast striking faults in relief, and different types of buried hills are distributed in certain order from the south gentle slope to the north abrupt slope, where burial depth almost has no influence on reservoir properties. The main source rock in Es3L and Es4U of Eogene and coal measure source rock in Carboniferous and Permian provide sufficient petroleum resources[14]. Bottom depth of petroleum charging is the low limit for reservoir forming, and buried hills above the source injection window are all favorable traps. By combining distribution of source rock with distribution of buried hills, 11 efficient layer exploration units have been confirmed with optimal selection method, which are predicted to contribute reserves of more than 1.5×108 t. Es4LEk were deposited during initial downfaulted period in Jiyang depression. Under arid climate and shallow water body conditions, there developed 3 series of widely distributed thick stable salt-gypsum beds. Under the good sealing of the salt-gypsum beds, Es4LEk forms an independent petroleum system. Saline environment promoted efficient hydrocarbon generation and expulsion[1422], so self-generated reservoir is the important direction for current and mid-term exploration of this layer. With self-generated condensate oil reservoirs as the major targets, and structural reservoirs sourced by other source rock as secondary targets, 9 layer exploration units have been selected as favorable targets, namely Ek in GuangliQingnan, Es4L in YanjiaYong'anzhen, etc.
6.
during stage of high exploration degree can sort out efficient orientation and drilling targets, and thus decrease exploration risks and increase profit in downfaulted basins. Layer exploration unit is the basic element for exploration practice during the stage of high exploration degree, and is also basic geologic unit with relatively complete structure, sedimentation, and accumulation system, which reflects the consistency between objective geologic conditions and subjective cognition. The concept of layer exploration unit provided new idea for exploration in mature area. Basic contents of layer exploration unit evaluation include reasonable division of remaining resources, economic evaluation, adaptability of present geologic model and exploration techniques, and evaluation of surface engineering technology. For the purpose of efficient exploration, 305 layer exploration units have been identified in Jiyang depression , among which 66 are deemed to be profitable, geologically reliable, technically feasible under current economic situation, and have surface condition suitable for exploration, and results of this study have provided good reference for the decisionmaking on exploration in the 13th Five Year Plan.
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