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Re-exploration program for petroleum-rich sags and its significance in Bohai Bay Basin, East China
PETROLEUM EXPLORATION AND DEVELOPMENT Volume 42, Issue 6, December 2015 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2015, 42(6): 790–801.
RESEARCH PAPER
Re-exploration program for petroleum-rich sags and its significance in Bohai Bay Basin, East China ZHAO Xianzheng1,*, WANG Quan1, JIN Fengming1, LUO Ning1, FAN Bingda1, LI Xin2, QIN Fengqi1, ZHANG Hongwei3 1. PetroChina Huabei Oilfield Company, Renqiu 062552, China; 2. PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China; 3. BGP Inc., China National Petroleum Corporation, Zhuozhou 072750, China
Abstract: A comprehensive analysis is done to investigate the exploration maturity and exploration potential in the petroleum-rich sags in the Bohai Bay Basin, indicating that the re-exploration program is of great significance. The advances in geological theories, key engineering technologies and exploration methods involved in the re-exploration program are described. Taking the Raoyang sag as an example, the study documents how the program was conducted and discusses its promising future of application. Petroleum-rich sags, with an oil resource abundance of greater than 20×104 t/km2 and a total oil resource of over 3×108 t, are the main domains for petroleum exploration and production in the Bohai Bay Basin. With a discovery maturity of more than 50%, these petroleum-rich sags have entered the high maturity exploration stage and discovery of new oil and gas reserves has become more and more difficult. However, the undiscovered oil resources in these sags are abundant and amount to 105×108 t, so that they are still the main exploration domains in the Bohai Bay Basin. With the petroleum-rich sags in the Jizhong depression of the Bohai Bay Basin as an example of the re-exploration program, the study presents a detailed documentation of oil accumulation in subsags, reservoir-forming in the subtle buried hills, and oil migration and accumulation in the weakly deformed structural slopes. It deals with the advances in the merged 3-D seismic prospecting over an entire sag, facies-based reservoir prediction, stimulation techniques for complex reservoirs and fast efficient drilling techniques. Furthermore, it proposes the re-exploration program, which comprises a new round of overall exploration philosophy, new evaluation and planning. It discusses the important roles played by the re-exploration program in sustaining the reserve additions in the highly explored petroleum-rich sags. Key words: Bohai Bay Basin; petroleum-rich sag; re-exploration program; theoretical cognition; technical approach; exploration practice
Introduction The oil-rich sags in the Bohai Bay Basin, one of the major petroliferous basins in east China, have favorable hydrocarbon accumulation conditions[12]. Since an industrial oil flow (8.1 ton/day oil production) was recorded from the Paleogene in Well Hua-8 in the Xinzhen structure of the Dongying sag on April 15th, 1961, large-scale hydrocarbon exploration has been lasting for more than 50 years in this basin[3]. By the end of 2013, its cumulative proved oil reserves in place reached to 140.52×108 t, accounting for 40% of the total proved oil reserves in China. Its oil output in 2013 was 7596×104 t, accounting for 25% of the total oil and gas production in China[4]. Thus, this basin takes a very important position in the development of Chinese petroleum industry. By the end of the 20th century, the exploration and development in the Bohai Bay Basin had reached a very high ma-
ture state, with the discovery maturity of in place oil and gas resources of 51.3% and 12.8%, respectively. With the rise of exploration maturity, the Bohai Bay Basin is facing the difficult situations that all the significant structures have been fully covered by 3-D seismic survey, drilling density is already high, and the discovered oil accumulations are becoming smaller and smaller. As a result, this basin has been slacking in oil and gas reserve additions, and decline in oil production, thus the further exploration and production in this basin is facing a number of challenges. Yuan Xuanjun and Qiao Hansheng et al. proposed that the sags with in place resources abundance of more than 20×104 t/km2 and in place oil resources of over 3×108 t in the Bohai Bay Basin could be classified as oil-rich sags[5]. The proved in place oil reserves and annual oil equivalent production of these oil-rich sags account for 82% and 73% of the total re-
Received date: 12 May 2015; Revised date: 29 Sep. 2015. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the PetroChina Science and Technology Major Project (2014E-35). Copyright © 2015, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
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serves and production in the Bohai Bay Basin, respectively. These sags have been the major targets in the Bohai Bay Basin, and also the major distribution areas of the remaining oil. How to deepen hydrocarbon exploration in these oil-rich sags in the Bohai Bay Basin has been a key subject that has been continuously studied by many researchers and explorationists. Jia Chengzao et al. proposed to carry out studies on some key techniques (such as seismic reservoir prediction technique and sequence stratigraphy technique etc. for exploring lithologicstratigraphic plays)[68]. Zhou Haimin et al. put forward “six fines” exploration method (fine implementation of 3-D seismic re-prospecting, fine study on oil field geology, fine selection of drilling patterns, etc.)[910]. Zhang Shanwen thought that “thinking out of box” is the key for finding oil in highly explored areas[11]. Meng Weigong discussed deepening of exploration from several aspects (thinking innovation, enhancing seismic data quality and developing leading technologies, etc.)[12]. Cai Jia and Chen Wenxue et al. discussed further exploration potentials of old blocks and old oil fields, on the basis of in-depth study of hydrocarbon enrichment patterns in these regions[1314]. Xu Changgui raised a new approach for re-exploration in offshore areas[15]: aiming at medium-large oil and gas fields, the approach took an overall research method and focused on innovating geological ideas and getting a better understanding of subsurface geological conditions, and was safeguarded by optimizing the technology portfolio and fully utilizing the existing information potentials. Deng Yunhua systematically analyzed the efficiency of re-exploration in the offshore regions of the Bohai Bay Basin from five aspects[16]. In conclusion, re-exploration has become an important new idea for finding new oil in oil-rich sags with a high exploration maturity. By analyzing the exploration matiurity and potential of oil-rich sags in the Bohai Bay Basin, this paper introduces the necessity of carrying out the re-exploration program in oil-rich sags, related geologic theories, key engineering techniques and exploration methods, and presents the progress of re-exploration in Raoyang sag as an example, to envision the broad application prospects of the re-exploration program in oil-rich sags.
1.
Geologic setting of the study area
Located in the east of North China and the south of Northeast China, the Bohai Bay Basin covers the Bohai Sea and its coastal areas. Adjacent to the Taihang Mountain in the west, the Yanshan Mountain in the north, the Liaodong hills and Shandong hills in the east, connected with North China Basin in the south, with the Liaohe Plain and the North China Plain in the center, it has an irregular rhombic shape, with an area of 20×104 km2 (Fig. 1). The Bohai Bay Basin is an extensional and superimposed Mesozoic-Cenozoic continental sedimentary basin, developed on the Middle-upper Proterozoic-Paleozoic sedimentary cover
within the North China platform. The Cenozoic is the major oil-bearing succession in this basin, with many faults and very complex structures. During the Paleogene time, it was mainly a rift. During the Neogene time, it was dominated by sagging. At the end of Neogene, this basin took its current configuration. A number of dominant faults developed within this basin divide it into eight sub-basins (Liaohe, Jizhong, Huanghua, Jiyang, Changwei, Linqing, Bozhong and Liaodongwan) and two uplifts (Cangxian and Chengning). These sub-basins can be further subdivided into 58 sags and 52 rises. Controlled by one major fault, each sag is a half-graben or has an asymmetric graben pattern. During the Paleogene rifting period, the major deposits are dominated by lacustrine facies and include four second order depositional cycles: Kongdian Formation (Ek), Member 4 (Es4)-Member 3 (Es3), Member 2 (Es2)-Member 1 (Es1) of Shahejie Formation, and Dongying Formation (Ed). During the Neogene sagging period, the major deposits consist of fluvial facies and include two second order depositional cycles: Guantao Formation (Ng) and Minghuazhen Formation (Nm). There are multiple exploration intervals in the Bohai Bay Basin. Since the oil discovery in Well Hua-8 in 1961, “source controlling theory” and “complex hydrocarbon accumulation play fairway theory” had been formed in the early stage, and large-scale exploration for structural traps in the Paleogene and Neogene and buried hills in the basement have been carried out. Since 2000, the new recognition of “sag-wide oil-bearing theory” in oil-rich sags has been gradually established[17], then the exploration for subtle oil and gas plays (lithologic and stratigraphic plays) has been continuously strengthened. To date, more than 10 oil and gas bearing intervals have been found in the Archean to Neogene. The major oil-bearing reservoir rocks are dominated by nonmarine sandstones. Others include marine carbonates and metamorphic rocks in the basement, and lacustrine carbonates and volcanic rocks.
2. Definition and necessity of the re-exploration program The re-exploration program is quite different from the primary exploration in terms of seismic data base, understanding of oil and gas accumulation, major exploration targets, engineering technologies and exploration methods. Taking oil-rich sags with a high exploration maturity and abundant remaining oil and gas resources as the research targets, based on 3-D seismic data covering the whole sag, guided by some new theories (“sag-wide oil-bearing theory”[17], “hydrocarbon accumulation in troughs”[1820]) for oil-rich sags, taking each sag as a research unit, taking all types of oil reservoirs (mainly lithologic and stratigraphic ones) as the objects, with the support of new engineering technologies (such as advanced and applicable well logging, drilling and reservoir stimulation techniques, etc.), the
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Fig. 1.
Distribution of major oil-rich sags in Bohai Bay Basin.
re-exploration program involves a new round of whole exploration process (including overall cognition, overall evaluation and overall arrangement) in these oil-rich sags[21], so as to realize continuous and large-scale reserve additions in oil-rich sags with a high exploration maturity. Based on analysis of the present situation of oil and gas resources in the Bohai Bay Basin, though the discovery maturity of this basin is higher, these oil-rich sags are still the major domains of the remaining oil and gas resources and still have a large exploration potential. There exist fourteen oil-rich sags (Raoyang sag, Baxian sag, Langgu sag, Qikou sag, Nanpu sag, Damintun sag, western Liaohe sag, eastern Liaohe sag, Dongying sag, etc.) in onshore and offshore parts of the Bohai Bay Basin, where the undiscovered oil in place resources may
be up to 50×108 t[5], thus these sags are still important areas for future production and reserve additions. Though oil-rich sags in the Bohai Bay Basin have entered the “two-high” stage (high discovery maturity and high exploration maturity), different exploration blocks, plays and domains still differ widely in terms of exploration maturity, specifically, (1) the exploration maturity of the structural play is higher, whereas that of stratigraphic and lithologic plays is lower; (2) the exploration maturity of structural high belts is higher, while that of sags and structural low belts is lower; (3) the exploration maturity of middle and shallow formations is higher, but that of deep-ultra deep formation is lower; (4) the exploration maturity of the hill-top structures at buried hill is higher, whereas that of the intra-buried hills and at buried hill
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slopes is lower; (5) the exploration maturity of siliciclastic sandstones of lacustrine facies is higher, whereas that of some other facies (such as subaqueous fan conglomerates, carbonates [22] and volcanic rocks) is lower; (6) the exploration maturity of conventional oil and gas reservoirs is higher, but that of tight oil and gas reservoirs is lower[2324]. By continuously deepening the investigation and enhancing exploration, the above blocks, plays and domains of low exploration maturity can become new important targets for new reserve additions. Oil-rich sags where re-exploration program is suitable should meet the following three conditions: (1) existing geological concepts or exploration techniques and methods can not yield new and important breakthroughs and progress; (2) the sag still has plentiful remaining resources; (3) the whole sag or the favorable exploration areas in the sag have been basically covered by merged 3-D seismic survey.
3. Advances in theories applied in re-exploration program 3.1. Concepts of “hydrocarbon accumulation in troughs” in continental rifts A trough in a continental rift refers to the deep zone and middle-lower parts of the slope except for positive structural belts in the rift. The troughs in places with weaker tectonic activities, are favorable regions for developing stratigraphic and lithologic traps[25]. The theory of “hydrocarbon accumulation in troughs” includes five core contents: (1) Multi-factor sand control. The distribution of sand-bodies in troughs is controlled by several factors (boundary fault style, structural trend type, systems tract, slope-break belt and sedimentary microfacies). (2) Preponderant hydrocarbon accumulation. Traps in troughs have some preponderances (early formation, early hydrocarbon charging and good preservation conditions). (3) Key-factor enrichment. Hydrocarbon enrichment in
Fig. 2.
troughs is controlled by three key factors (hydrocarbon generation intensity threshold, major migration pathway and critical scale of reservoir body). (4) Paragenesis and complementation. The distribution of oil and gas in structural plays and stratigraphic-lithologic plays in troughs is characterized by the pattern of paragenesis and complementation. (5) Multiple patterns. The troughs have favorable conditions for forming multiple types of stratigraphic-lithologic oil accumulations (Fig. 2). The new theory and cognition of “hydrocarbon accumulation in troughs” in continental rifts have increased the hydrocarbon exploration value of trough regions, and laid a theoretical foundation for realizing overall evaluation and whole sag exploration in rifts. Guided by it, several play fairways with substantial reserves have been found in the Maxi and Hejian troughs in the Raoyang sag and the Liuquan trough in the Langgu sag in the Bohai Bay Basin (Fig. 3). 3.2. Concept of hydrocarbon accumulation in subtle buried hill reservoirs With the rise of the exploration maturity, it is more and more difficult to find large-scale new reserves. Subtle buried hill reservoirs mainly include three types: subtle deep buried hill, intra-buried hill and buried hill slope. They are the dominant targets for deepening exploration in the buried hill play in oil-rich sags[2627]. Through in-depth study of hydrocarbon accumulation conditions and major controlling factors of subtle buried hill reservoirs, three new cognitions have been achieved: (1) The subtle deep buried hills below 3 500 m depth with Es3-Es4 as the major source rocks, are relatively abundant in oil and gas resources. With caverns and fissures as main storage spaces, and less affected by burial depth in petrophysical properties, they have a better storage capacity. (2) There are six sets of major reservoir-cap assemblages
Hydrocarbon accumulation patterns in troughs of a rift.
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Fig. 3.
Exploration results in Jizhong depression.
(Wumishan Formation in Jixian System, Fujunshan Formation-Mantou Formation or Xuzhuang Formation in Cambrian System) in the Proterozoic-lower Paleozoic. The intra-buried hills have favorable hydrocarbon accumulation conditions. (3) Hydrocarbon accumulation in subtle buried hill reservoirs is mainly controlled by two geologic factors: transporting capacity of hydrocarbon migration pathways, petrophysical properties of reservoir intervals in deep buried hills and intra-buried hills[28].
Directed by the above new cognitions of hydrocarbon accumulation in subtle buried hill reservoirs, ten subtle buried hill oil accumulations (Chang 3, Ninggu 8x, Hu 8, Niudong 1, Wengu 3 etc.) with high production and high efficiency have been successively discovered in the Changyangdian buried hill trend, Suning buried hill trend and Sunhu buried hill trend in the Raoyang sag, Niudong buried hill trend and Wenan slope belt in the Baxian sag in the Bohai Bay Basin, showing broad exploration prospects of buried hill oil reservoirs.
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3.3. Concpepts of hydrocarbon accumulation in large-scale stratigraphic-lithologic reservoirs on slopes of weak structures The slope belt is one of the main hydrocarbon accumulation zones in oil-rich sags. Early exploration mainly focused on local structural oil reservoirs in the nose structural background on slopes, and the discovered oil reservoirs generally are distributed in point pattern or in some local areas. In the past few years, by deepening the study on hydrocarbon accumulation conditions in slope belts represented by Lixian slope and Wenan slope in Jizhong depression in the Bohai Bay Basin, we have reached some new findings, among them, three main cognitions are: (1) Lixian slope and Wenan slope are located in weak structural belts, both are structural-deposition slope type, with few local structures, which is favorable for forming stratigraphic-lithologic oil reservoirs[29]. (2) Hydrocarbon migration and accumulation were jointly controlled by both transporting capacity of faults and physical properties of reservoir sand bodies. Experiment results of hydrocarbon accumulation modeling indicate that when transporting capacity of faults is low, hydrocarbons migrate in several pathways, likely forming multiple oil-bearing layers in low positions (Fig. 4); when transporting capacity of faults is high, hydrocarbons migrate along predominant pathways with good physical properties to higher locations of the slopes in step style, thus oil-bearing layers only distribute in some local regions (Fig. 4). (3) We have established new modes of hydrocarbon accumulation in the above two types of weak structures. In Wenan slope, there is no oil source rock basically, but well-developed sand bodies, the hydrocarbons there came from the major oil generation trough of adjacent Baxian sag. Faults, sand bodies and unconformities make up the composite conducting pathways. Hydrocarbons mainly migrated to distal positions in step style. Accordingly, we have established several new modes of hydrocarbon accumulation (such as shallow structure-river channel sand in outer belt of the slopes, delta front sand bodies in inner belt of the slopes, etc.). In contrast, in Lixian slope, there develop a hydrocarbon generation layer (mainly the lower member of Es1), but the sand bodies are poorly developed, hydrocarbons there mainly migrate to proximal positions near source rocks, which is favorable for forming several types of oil reservoirs (such as faulted nose sand lens with low amplitude and updip pinch-out sand bodies, etc.). Guided by the above study, we have carried out overall evaluation on Lixian slope and Wenan slope, and newly found oil reservoirs have features of vertical superimposition (intervals) in vertical direction and continuous distribution on plane, forming two large-scale monolith reserve zones of 108 t order.
4. Key technologies and practices of re-exploration program In the implementation process of re-exploration program in oil-rich sags, the exploration targets have apparently changed
Fig. 4. Geologic model and physical modeling results of hydrocarbon migration in slope belt.
from structural plays to stratigraphic-lithologic plays, and from thick sand oil reservoirs to thin interbedded sand oil reservoirs. 4.1. Merged 3-D seismic prospecting technique in whole sags In the new round of overall study of oil-rich sags, we firstly evaluated and classified the quality of old 3-D seismic data, then repeated a new 3-D seismic acquisition in favorable hydrocarbon accumulation areas and carried out 3-D seismic acquisition over areas not covered in the previous survey; after that, we have conducted an overall merging of fine 3-D
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seismic processing[30]. In order to merge 3-D seismic data of different surveys, we adopted a unified definition of coordinate system and bin homogenization in the whole area. During the merging processing, we utilized some key techniques, such as establishment of near surface model, multi-domain fine pre-stack denoising, merged high-resolution velocity modeling, consistency processing of amplitude, phase and frequency, and moveout correction processing etc. In the overall merged processing of 3-D seismic data in oil-rich sags in the Jizhong depression in the Bohai Bay Basin, we developed pre-stack depth migration processing technique for mass data in super-large areas, established a super computer group (including 30 000 cores, with running speed of 1×1015 times/second), realized migration processing to 105 TB mass data (equal to 200 portable computers with 500 GB), and provided computation services with a high efficiency and high performance for overall merged 3-D seismic processing. By using the above techniques and methods, the differences in seismic data amplitude, phase and frequency between various surveys were eliminated, the merged 3-D seismic data set has a much better imaging for delineating faults. By adopting these techniques and methods, we have estab-
Fig. 5.
lished an overall merged 3-D seismic data set covering some oil-rich sags (Langgu, Baxian and Raoyang, etc.) in the Jizhong depression in the Bohai Bay Basin, with an area of 1×104 km2 (Fig. 3). This lays a solid data foundation for establishing high-resolution stratigraphic sequence frameworks, in-depth study of structures and fine delineation of sand bodies etc. in the re-exploration program of oil-rich sags. 4.2. Facies-based fine prediction technique for reservoirs As the major exploration targets in the re-exploration program of oil-rich sags are stratigraphic-lithologic traps, more sophisticated reservoir prediction techniques with seismic data are requited. Through years of study and practice, we have developed facies-based geological-seismic reservoir prediction technique (Fig. 5). Based on high-resolution 3-D seismic data, this technique involves establishing the structural-stratigraphic framework, classifying various types of depositional systems by combining geologic facies with seismic facies, dividing prediction into early stage without well/with few wells and high-accuracy stage with more wells, predicting lithologies, sand bodies, petrophysical properties and hydrocarbon bearing properties
Technical flow chart of facies-based geologic-seismic reservoir prediction.
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with the optimum geophysical methods selected, and finally, making comprehensive evaluation of all these prediction results to pick out reservoir prediction results coinciding with the reality to direct exploration deployment. In this technique, the constraints of the sequence model, structure model and deposition model are the keys for facies-based reservoir prediction, which is helpful for improving the accuracy of reservoir prediction; frequency spectrum decomposition, cooperative inversion of acoustic impedance and reservoir parameters, merging amplitude and frequency, etc. techniques can improve the accuracy of prediction of thin reservoirs. We have applied the above methods and techniques to finely delineate spatial distribution of reservoir beds in variable and complex sedimentary facies, which can improve drilling success. There developed fluvial channel sand bodies in the outer belt of the Wenan slope in Baxian sag, and fluvial channel sand bodies in delta front facies in Es3 in its inner belt. We utilized faciesbased fine prediction techniques for reservoir beds to describe spatial distribution of reservoir sand bodies there. Out of 39 wells deployed according to our study, 22 yielded industrial oil flows, with a success rate of exploratory wells of 56.4%, which is 16% higher than that in 2006, achieving an overall exploration breakthrough in inner and outer belts, in multiple layers and in multiple domains. 4.3.
Stimulation techniques for complex reservoirs
In the process of implementing re-exploration program in oil-rich sags, some new domains (such as oil reservoirs in (extra-) deep buried hills and intra-buried hills, oil reservoirs with low porosity and low permeability, tight oil reservoirs, etc.) have gradually become important exploration targets. Accordingly, some corresponding advanced and applicable exploration techniques (fracturing stimulation technique for carbonate reservoir beds in deep buried hills and sand reservoir beds with low porosity and low permeability, volume fracturing stimulation technique for tight oil, etc.) have been formulated or developed. By promotion and application, oil and gas production in individual wells has been greatly improved, and oil reservoir performance has been raised. The bottom-hole temperature in the Niudong buried hill oil reservoir in the Baxian sag is up to 201 C. Aiming at the problem of high temperature and too rapid acid-rock reaction (which is unfavorable for creating long fractures), we have developed the VES clean acid formula (acting as host acid in acid fracturing) with a high viscosity, super high temperature resistance and stable performance, and created a volume fracturing stimulation technique (prefixing exploring fractures, using main fractures for communication and auxiliary fractures for supplement, diversion of fracturing fluid, multiple stage injection, closed acidizing) to conduct large-scale acid fracturing with multiple stages, achieving network volume stimulation effect (main fractures act as high speed channels, auxiliary fracture nets act as supplement)[31]. After fracturing, the well
yielded a daily oil production of 642.91 m3, and daily gas production of 5.62×104 m3, marking it the deepest oil in carbonate buried hills with the highest temperature in eastern China. For the Lixian slope in the Raoyang sag, aiming at tricky issues such as fine lithology, low hardness, thin interbedded layers, etc., we have conducted analysis of oil reservoir performance, study of layer selection for fracturing and optimization of technique parameters, come up with a new fracturing strategy of rational scale, moderate sand ratio, anti-swelling in the whole process, moderate viscosity reducing[32]. The new fracturing strategy has been used for 11 well times, with a success ratio of 92%, resulting in oil production increase from 0.031.83 t/d to 4.2926.72 t/d. The effect is apparent, and oil reservoirs and reserves quality have been improved. 4.4.
Quick and efficient support drilling techniques
In an ultra-deep buried hill (Niudong 1 buried hill) at the western slope belt in the Baxian sag in the Jizhong depression in the Bohai Bay Basin, the technique combination of formate drilling fluid with strong inhibition and high temperature resistance, special drill bit, downhole pressure impulse generator, rotary blowout plug at the well head to control pressure, oil-in-water low-density drilling fluid was used to realize low-pressure underbalanced drilling at 6 000 m well depth. The technique combination resulted in an average rate of penetration (ROP) increase of 14.7%, average drilling footage of individual drill bits 6.3 times as much as previous record, fewer drill bits used and less tripping times, which shortened drilling cycles, reduced exploration cost, protected oil and gas layers, and achieved fast and safe drilling. Four drilling platforms in the Guan structure in the Langgu sag were set up to conduct multi-lateral and extended reach drilling. Among them, the Gu 43 platform has supported drilling of more than 10 exploratory wells, which made it possible to drill in urban areas, and effectively promoted exploration discoveries in the Langgu sag. 4.5.
Practice of re-exploration program
The Raoyang sag, which lies the central part of the Jizhong depression in the Bohai Bay Basin and covers an area of 5 280 km2, is taken as an example to illustrate. According to the results of a new round of hydrocarbon resources evaluatoin, initally in-place oil resources in the Raoyang sag are 11.7×108 t. As of 2005, its proven in-place oil reserves were 6.52×108 t, accounting for 68.2% of the total proven in-place oil reserves (9.56×108 t) in the Jizhong depression; its remaining in-place oil resources are 5.18×108 t, accounting for 34% of the total remaining in place oil resources in the Jizhong depression, clearly, it is an oil-rich sag with the biggest exploration potential in the Jizhong depression. Based on the abundant remaining oil and gas resources in the Raoyang sag, the re-exploration program with innovated exploration methods (Fig. 6) resulted in large-scale reserve additions in several blocks and multiple domains.
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Fig. 6. Flow chart of re-exploration program in oil-rich sags in rift basins.
4.5.1. sag
Setting up of 3-D seismic data base for the whole
Since 2006, based on analysis of old 3-D seismic data quality, we conducted an overall deployment and step by step new acquisition of 3-D seismic data over 11 blocks covering an area of 2 297 km2; launched massive merged 3-D seismic processing, and set up 3 415 km2 integrated 3-D seismic data volume. By eliminnating the differences in amplitude, frequency and phase among different 3-D seismic surveys, the massive merged 3-D seismic data has a clear composite wave features, accurate fault imaging and distinct fault structures, setting a fine 3-D seismic data base for conducting sequence stratigraphic analysis, structural study, local structure confirmation, and analysis of deposition systems and sand bodies in this sag. 4.5.2. Reconstruction of structural configuration and depositional history as well as reservoir architecture in the sag (1) Structure reconstruction. By 3-D structure interpretation in the whole sag, we realized the change from previous line interpretation in blocks to full 3-D interpretation in sags, then our understanding of structural belts and structural styles are more systematic and complete; especially, our understanding of structural features of buried hills are deepened. In the Changyangdian buried hill belt, our work changed from confirming traps on the buried hill top to finely confirming structures on the Wumishan Formation top, finding several new buried hill traps (Chang 3, Chang 6, etc.). In the Suning buried hill belt, we found the Ninggu 8 buried hill trap (with a local trap area of 6 km2), providing favorable exploration targets for buried hill exploration. (2) Deposition reconstruction. Based on merged 3-D seismic data, we carried out fine sequence stratigraphic division and correlation in the whole sag. After that, we examined the depositional systems of 10 third-order sequences and 30 systems tracts in the Ek-Es formations carefully, completing fine
industrial mapping of systems tracts for each sequence in the whole sag, and finely describing the distribution scopes of favorable sedimentary facies belts in various systems tracts. (3) Reservoir reconstruction. With an emphasis on deep (Es3-Es4) reservoir intervals, we rechecked the reservoir beds. The study results show deep clastic reservoirs are characterized by coexistence of primary and secondary pores. We confirmed a lower depth limit of predominant reservoir exploration (700 m deeper than the previous lower limit), greatly expanding exploration domain. Moreover, by overlapping four types of maps (sedimentary facies, diagenetic system, diagenetic stage and formation pressure), we picked out several favorable areas of deep reservoir beds, including the Ma 99 well block in the Maxi trough, the Ninggu 10 well block in the Hejian trough, the Lu 44 well block in the Liuxi trough, the Qiangshen 1 and Huang 3 well blocks in the Raonan trough, etc. 4.5.3. Quantitative characterization of the spatial distribution of oil and gas resources; establishment of new hydrocarbon accumulation models in various domains Based on fine classification and evaluation of source rocks with well logging data, we modeled hydrocarbon migration and accumulation processes in this sag, described hydrocarbon migration and accumulation processes in various geological periods and spatial distribution of hydrocarbon resources in various blocks and stratigraphic intervals (Fig. 7), and predicted the major distribution areas of the remaining hydrocarbon resources. On the basis of the above study, we made a comprehensive analysis of hydrocarbon accumulation conditions in various blocks and domains (such as trough region, slope belt and buried hill, etc.), and established hydrocarbon accumulation models in various domains in the Raoyang sag. In the Maxi-Liuxi troughs, we set up new models of sand oil reservoirs in delta front facies, and sand oil reservoirs in fluvialchannels by fault and sand coupling, etc. For the Lixian slope, we established a new reservoir formation model of low-amplitude nose structure sand lens and sand updip pinch-out. For subtle buried hills, we built three kinds of hydrocarbon accumulation models in buried hills: “old reservoir-old sealing coupling” (Chang 3), “red cap rock-lateral migration” (Ninggu 8x), and “big hill-crest accumulation” (Hu 8)[26,33]. These results played an important role in the overall investigation, trap discovery and confirmation, favorable target evaluation and selection of prospects in the above blocks and domains. Directed by the research results of the overall understanding and overall evaluation, with stratigraphic-lithologic oil reservoirs and subtle buried hill oil reservoirs as the major targets, we implemented an overall deployment and conducted the re-exploration program in steps in some favorable blocks (Lixian slope, Maxi-Hejian troughs, Changyangdian buried hill belt, Suning buried hill belt and Sunhu buried hill belt, etc.) in Raoyang sag, achieving important hydrocarbon
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Fig. 7.
Prediction of oil and gas distribution in major layers of Raoyang sag.
exploration results. The new 3P in place oil reserves are 2×108 t (5 727×104 t proven reserves), a total new production capacity of 114.5×104 t, realizing large-scale production and reserves increase. With the above methods, we implemented re-exploration program in some oil-rich sags (Baxian and Langgu, etc.), achieving major discoveries of in-place oil and gas reserves with a magnitude of 108 t. In the re-exploration program in the Baxian sag, we established composite oil reservoir model (including Es1-Ng low-amplitude structure-fluvial channel sand lithology) in the outer belt of the Wenan slope, lithologic oil reservoir model in delta front sand bodies controlled by structure-sedimentary slope break in the inner belt of the Wenan slope, and hydrocarbon accumulation model in extra-deep buried hill in the Niudong fault terrace in the steep slope belt in the western Raoyang sag. The re-exploration program is fruitful, bringing about new exploration discoveries of 1.15×108 t new 3P in-place oil reserves, and the discovery of the extra-deep and high temperature buried hill oil and gas reservoir in the Wumishan Formation in the Niudong 1 Field. The application of the above re-exploration methods in the exploration in the Aer sag (an oil-rich sag in the Erlian Basin) sped up its exploration pace. Since exploration drilling began in this sag in 2008, thick and high quality source rocks were found in the Well Aer-1, and high-productivity industrial oil flow was obtained in the Well Aer-3. By the year of 2010, a total of 10 469×104 t were added to the probable and possible in place oil and the discovery was named as the Aer Oilfield. It took only three years from drilling the first well to the discovery of the new oil field. Compared with similar sags, its exploration cycle is shortened by 5-10 years, achieving scientific, efficient and quick exploration of the new sag. Currently, with a 40×104 t annual production capacity, the Aer Oilfield has become an important oil and gas production base in the
Erlian Basin, contributing greatly to the steady increase of oil production in the basin[34]. The implementation of re-exploration program in oil-rich sags in the Jizhong depression in the Bohai Bay Basin and the Erlian Basin has brought about new breakthroughs and large discoveries in oil and gas exploration successively. Since 2009, the reserve addition has been kept at 108 t or so per year (Fig. 8), achieving positive results in oil and gas exploration, and blazing a new trail of deepening exploration in mature explored areas. Currently, the discovery maturities of oil resources in oil-rich sags (Raoyang sag, western Liaohe sag, Qikou sag, Dongying sag and Bozhong sag, etc.) in the Bohai Bay Basin are generally between 40%60%, representing a high exploration maturity. However, in the above oil-rich sags, the remaining in-place oil resources are up to 105×108 t, accounting for 78% of the remaining in-place oil resources in the Bohai Bay Basin, thus they have resources potentials for continuous and deepening exploration. In the Jiyang depression, through continuous and deepening investigation of hydrocarbon accumulation patterns in oil-rich sags, efficient exploration has been continued for 32 years since 1983, with a new proved in-place oil reserve of 1×108 t/year. This means that oil-rich sags in old explored areas are still important exploration regions for discovering large scale reserves.
5.
Conclusions
Oil-rich sags in the Bohai Bay Basin play an important role in oil and gas exploration and production in China. Still abundant in remaining oil and gas resources, they vary widely in exploration maturity, are the major targets for oil and gas production and reserves addition, thus, it is necessary to carry out re-exploration program in these sags. As a complex and systematic project, re-exploration program in oil-rich sags
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Fig. 8.
Total oil reserve (3P) growth in Jizhong depression in Bohai Bay Basin and Erlian Basin.
must be guided by new concepts on hydrocarbon accumulation, supported by advanced and applicable new exploration (engineering) techniques, to conduct the overall investigation, overall evaluation and overall arrangement. Oil and gas exploration practices show that implementation of the re-exploration program in oil-rich sags can realize large-scale new oil and gas reserve additions in mature explored areas. Moreover, the ideas and methods involved in the re-exploration also have practical guiding significance for oil and gas exploration in newly found oil-rich sags. Therefore, we should constantly renew our geologic concepts, positively promote technical progress, and carry out re-exploration in oil-rich sags vigorously, so as to promote new progress of oil and gas exploration in oil-rich sags in the Bohai Bay Basin.
[7]
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RESEARCH PAPER
Re-exploration program for petroleum-rich sags and its significance in Bohai Bay Basin, East China ZHAO Xianzheng1,*, WANG Quan1, JIN Fengming1, LUO Ning1, FAN Bingda1, LI Xin2, QIN Fengqi1, ZHANG Hongwei3 1. PetroChina Huabei Oilfield Company, Renqiu 062552, China; 2. PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China; 3. BGP Inc., China National Petroleum Corporation, Zhuozhou 072750, China
Abstract: A comprehensive analysis is done to investigate the exploration maturity and exploration potential in the petroleum-rich sags in the Bohai Bay Basin, indicating that the re-exploration program is of great significance. The advances in geological theories, key engineering technologies and exploration methods involved in the re-exploration program are described. Taking the Raoyang sag as an example, the study documents how the program was conducted and discusses its promising future of application. Petroleum-rich sags, with an oil resource abundance of greater than 20×104 t/km2 and a total oil resource of over 3×108 t, are the main domains for petroleum exploration and production in the Bohai Bay Basin. With a discovery maturity of more than 50%, these petroleum-rich sags have entered the high maturity exploration stage and discovery of new oil and gas reserves has become more and more difficult. However, the undiscovered oil resources in these sags are abundant and amount to 105×108 t, so that they are still the main exploration domains in the Bohai Bay Basin. With the petroleum-rich sags in the Jizhong depression of the Bohai Bay Basin as an example of the re-exploration program, the study presents a detailed documentation of oil accumulation in subsags, reservoir-forming in the subtle buried hills, and oil migration and accumulation in the weakly deformed structural slopes. It deals with the advances in the merged 3-D seismic prospecting over an entire sag, facies-based reservoir prediction, stimulation techniques for complex reservoirs and fast efficient drilling techniques. Furthermore, it proposes the re-exploration program, which comprises a new round of overall exploration philosophy, new evaluation and planning. It discusses the important roles played by the re-exploration program in sustaining the reserve additions in the highly explored petroleum-rich sags. Key words: Bohai Bay Basin; petroleum-rich sag; re-exploration program; theoretical cognition; technical approach; exploration practice
Introduction The oil-rich sags in the Bohai Bay Basin, one of the major petroliferous basins in east China, have favorable hydrocarbon accumulation conditions[12]. Since an industrial oil flow (8.1 ton/day oil production) was recorded from the Paleogene in Well Hua-8 in the Xinzhen structure of the Dongying sag on April 15th, 1961, large-scale hydrocarbon exploration has been lasting for more than 50 years in this basin[3]. By the end of 2013, its cumulative proved oil reserves in place reached to 140.52×108 t, accounting for 40% of the total proved oil reserves in China. Its oil output in 2013 was 7596×104 t, accounting for 25% of the total oil and gas production in China[4]. Thus, this basin takes a very important position in the development of Chinese petroleum industry. By the end of the 20th century, the exploration and development in the Bohai Bay Basin had reached a very high ma-
ture state, with the discovery maturity of in place oil and gas resources of 51.3% and 12.8%, respectively. With the rise of exploration maturity, the Bohai Bay Basin is facing the difficult situations that all the significant structures have been fully covered by 3-D seismic survey, drilling density is already high, and the discovered oil accumulations are becoming smaller and smaller. As a result, this basin has been slacking in oil and gas reserve additions, and decline in oil production, thus the further exploration and production in this basin is facing a number of challenges. Yuan Xuanjun and Qiao Hansheng et al. proposed that the sags with in place resources abundance of more than 20×104 t/km2 and in place oil resources of over 3×108 t in the Bohai Bay Basin could be classified as oil-rich sags[5]. The proved in place oil reserves and annual oil equivalent production of these oil-rich sags account for 82% and 73% of the total re-
Received date: 12 May 2015; Revised date: 29 Sep. 2015. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the PetroChina Science and Technology Major Project (2014E-35). Copyright © 2015, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
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serves and production in the Bohai Bay Basin, respectively. These sags have been the major targets in the Bohai Bay Basin, and also the major distribution areas of the remaining oil. How to deepen hydrocarbon exploration in these oil-rich sags in the Bohai Bay Basin has been a key subject that has been continuously studied by many researchers and explorationists. Jia Chengzao et al. proposed to carry out studies on some key techniques (such as seismic reservoir prediction technique and sequence stratigraphy technique etc. for exploring lithologicstratigraphic plays)[68]. Zhou Haimin et al. put forward “six fines” exploration method (fine implementation of 3-D seismic re-prospecting, fine study on oil field geology, fine selection of drilling patterns, etc.)[910]. Zhang Shanwen thought that “thinking out of box” is the key for finding oil in highly explored areas[11]. Meng Weigong discussed deepening of exploration from several aspects (thinking innovation, enhancing seismic data quality and developing leading technologies, etc.)[12]. Cai Jia and Chen Wenxue et al. discussed further exploration potentials of old blocks and old oil fields, on the basis of in-depth study of hydrocarbon enrichment patterns in these regions[1314]. Xu Changgui raised a new approach for re-exploration in offshore areas[15]: aiming at medium-large oil and gas fields, the approach took an overall research method and focused on innovating geological ideas and getting a better understanding of subsurface geological conditions, and was safeguarded by optimizing the technology portfolio and fully utilizing the existing information potentials. Deng Yunhua systematically analyzed the efficiency of re-exploration in the offshore regions of the Bohai Bay Basin from five aspects[16]. In conclusion, re-exploration has become an important new idea for finding new oil in oil-rich sags with a high exploration maturity. By analyzing the exploration matiurity and potential of oil-rich sags in the Bohai Bay Basin, this paper introduces the necessity of carrying out the re-exploration program in oil-rich sags, related geologic theories, key engineering techniques and exploration methods, and presents the progress of re-exploration in Raoyang sag as an example, to envision the broad application prospects of the re-exploration program in oil-rich sags.
1.
Geologic setting of the study area
Located in the east of North China and the south of Northeast China, the Bohai Bay Basin covers the Bohai Sea and its coastal areas. Adjacent to the Taihang Mountain in the west, the Yanshan Mountain in the north, the Liaodong hills and Shandong hills in the east, connected with North China Basin in the south, with the Liaohe Plain and the North China Plain in the center, it has an irregular rhombic shape, with an area of 20×104 km2 (Fig. 1). The Bohai Bay Basin is an extensional and superimposed Mesozoic-Cenozoic continental sedimentary basin, developed on the Middle-upper Proterozoic-Paleozoic sedimentary cover
within the North China platform. The Cenozoic is the major oil-bearing succession in this basin, with many faults and very complex structures. During the Paleogene time, it was mainly a rift. During the Neogene time, it was dominated by sagging. At the end of Neogene, this basin took its current configuration. A number of dominant faults developed within this basin divide it into eight sub-basins (Liaohe, Jizhong, Huanghua, Jiyang, Changwei, Linqing, Bozhong and Liaodongwan) and two uplifts (Cangxian and Chengning). These sub-basins can be further subdivided into 58 sags and 52 rises. Controlled by one major fault, each sag is a half-graben or has an asymmetric graben pattern. During the Paleogene rifting period, the major deposits are dominated by lacustrine facies and include four second order depositional cycles: Kongdian Formation (Ek), Member 4 (Es4)-Member 3 (Es3), Member 2 (Es2)-Member 1 (Es1) of Shahejie Formation, and Dongying Formation (Ed). During the Neogene sagging period, the major deposits consist of fluvial facies and include two second order depositional cycles: Guantao Formation (Ng) and Minghuazhen Formation (Nm). There are multiple exploration intervals in the Bohai Bay Basin. Since the oil discovery in Well Hua-8 in 1961, “source controlling theory” and “complex hydrocarbon accumulation play fairway theory” had been formed in the early stage, and large-scale exploration for structural traps in the Paleogene and Neogene and buried hills in the basement have been carried out. Since 2000, the new recognition of “sag-wide oil-bearing theory” in oil-rich sags has been gradually established[17], then the exploration for subtle oil and gas plays (lithologic and stratigraphic plays) has been continuously strengthened. To date, more than 10 oil and gas bearing intervals have been found in the Archean to Neogene. The major oil-bearing reservoir rocks are dominated by nonmarine sandstones. Others include marine carbonates and metamorphic rocks in the basement, and lacustrine carbonates and volcanic rocks.
2. Definition and necessity of the re-exploration program The re-exploration program is quite different from the primary exploration in terms of seismic data base, understanding of oil and gas accumulation, major exploration targets, engineering technologies and exploration methods. Taking oil-rich sags with a high exploration maturity and abundant remaining oil and gas resources as the research targets, based on 3-D seismic data covering the whole sag, guided by some new theories (“sag-wide oil-bearing theory”[17], “hydrocarbon accumulation in troughs”[1820]) for oil-rich sags, taking each sag as a research unit, taking all types of oil reservoirs (mainly lithologic and stratigraphic ones) as the objects, with the support of new engineering technologies (such as advanced and applicable well logging, drilling and reservoir stimulation techniques, etc.), the
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Fig. 1.
Distribution of major oil-rich sags in Bohai Bay Basin.
re-exploration program involves a new round of whole exploration process (including overall cognition, overall evaluation and overall arrangement) in these oil-rich sags[21], so as to realize continuous and large-scale reserve additions in oil-rich sags with a high exploration maturity. Based on analysis of the present situation of oil and gas resources in the Bohai Bay Basin, though the discovery maturity of this basin is higher, these oil-rich sags are still the major domains of the remaining oil and gas resources and still have a large exploration potential. There exist fourteen oil-rich sags (Raoyang sag, Baxian sag, Langgu sag, Qikou sag, Nanpu sag, Damintun sag, western Liaohe sag, eastern Liaohe sag, Dongying sag, etc.) in onshore and offshore parts of the Bohai Bay Basin, where the undiscovered oil in place resources may
be up to 50×108 t[5], thus these sags are still important areas for future production and reserve additions. Though oil-rich sags in the Bohai Bay Basin have entered the “two-high” stage (high discovery maturity and high exploration maturity), different exploration blocks, plays and domains still differ widely in terms of exploration maturity, specifically, (1) the exploration maturity of the structural play is higher, whereas that of stratigraphic and lithologic plays is lower; (2) the exploration maturity of structural high belts is higher, while that of sags and structural low belts is lower; (3) the exploration maturity of middle and shallow formations is higher, but that of deep-ultra deep formation is lower; (4) the exploration maturity of the hill-top structures at buried hill is higher, whereas that of the intra-buried hills and at buried hill
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slopes is lower; (5) the exploration maturity of siliciclastic sandstones of lacustrine facies is higher, whereas that of some other facies (such as subaqueous fan conglomerates, carbonates [22] and volcanic rocks) is lower; (6) the exploration maturity of conventional oil and gas reservoirs is higher, but that of tight oil and gas reservoirs is lower[2324]. By continuously deepening the investigation and enhancing exploration, the above blocks, plays and domains of low exploration maturity can become new important targets for new reserve additions. Oil-rich sags where re-exploration program is suitable should meet the following three conditions: (1) existing geological concepts or exploration techniques and methods can not yield new and important breakthroughs and progress; (2) the sag still has plentiful remaining resources; (3) the whole sag or the favorable exploration areas in the sag have been basically covered by merged 3-D seismic survey.
3. Advances in theories applied in re-exploration program 3.1. Concepts of “hydrocarbon accumulation in troughs” in continental rifts A trough in a continental rift refers to the deep zone and middle-lower parts of the slope except for positive structural belts in the rift. The troughs in places with weaker tectonic activities, are favorable regions for developing stratigraphic and lithologic traps[25]. The theory of “hydrocarbon accumulation in troughs” includes five core contents: (1) Multi-factor sand control. The distribution of sand-bodies in troughs is controlled by several factors (boundary fault style, structural trend type, systems tract, slope-break belt and sedimentary microfacies). (2) Preponderant hydrocarbon accumulation. Traps in troughs have some preponderances (early formation, early hydrocarbon charging and good preservation conditions). (3) Key-factor enrichment. Hydrocarbon enrichment in
Fig. 2.
troughs is controlled by three key factors (hydrocarbon generation intensity threshold, major migration pathway and critical scale of reservoir body). (4) Paragenesis and complementation. The distribution of oil and gas in structural plays and stratigraphic-lithologic plays in troughs is characterized by the pattern of paragenesis and complementation. (5) Multiple patterns. The troughs have favorable conditions for forming multiple types of stratigraphic-lithologic oil accumulations (Fig. 2). The new theory and cognition of “hydrocarbon accumulation in troughs” in continental rifts have increased the hydrocarbon exploration value of trough regions, and laid a theoretical foundation for realizing overall evaluation and whole sag exploration in rifts. Guided by it, several play fairways with substantial reserves have been found in the Maxi and Hejian troughs in the Raoyang sag and the Liuquan trough in the Langgu sag in the Bohai Bay Basin (Fig. 3). 3.2. Concept of hydrocarbon accumulation in subtle buried hill reservoirs With the rise of the exploration maturity, it is more and more difficult to find large-scale new reserves. Subtle buried hill reservoirs mainly include three types: subtle deep buried hill, intra-buried hill and buried hill slope. They are the dominant targets for deepening exploration in the buried hill play in oil-rich sags[2627]. Through in-depth study of hydrocarbon accumulation conditions and major controlling factors of subtle buried hill reservoirs, three new cognitions have been achieved: (1) The subtle deep buried hills below 3 500 m depth with Es3-Es4 as the major source rocks, are relatively abundant in oil and gas resources. With caverns and fissures as main storage spaces, and less affected by burial depth in petrophysical properties, they have a better storage capacity. (2) There are six sets of major reservoir-cap assemblages
Hydrocarbon accumulation patterns in troughs of a rift.
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Fig. 3.
Exploration results in Jizhong depression.
(Wumishan Formation in Jixian System, Fujunshan Formation-Mantou Formation or Xuzhuang Formation in Cambrian System) in the Proterozoic-lower Paleozoic. The intra-buried hills have favorable hydrocarbon accumulation conditions. (3) Hydrocarbon accumulation in subtle buried hill reservoirs is mainly controlled by two geologic factors: transporting capacity of hydrocarbon migration pathways, petrophysical properties of reservoir intervals in deep buried hills and intra-buried hills[28].
Directed by the above new cognitions of hydrocarbon accumulation in subtle buried hill reservoirs, ten subtle buried hill oil accumulations (Chang 3, Ninggu 8x, Hu 8, Niudong 1, Wengu 3 etc.) with high production and high efficiency have been successively discovered in the Changyangdian buried hill trend, Suning buried hill trend and Sunhu buried hill trend in the Raoyang sag, Niudong buried hill trend and Wenan slope belt in the Baxian sag in the Bohai Bay Basin, showing broad exploration prospects of buried hill oil reservoirs.
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3.3. Concpepts of hydrocarbon accumulation in large-scale stratigraphic-lithologic reservoirs on slopes of weak structures The slope belt is one of the main hydrocarbon accumulation zones in oil-rich sags. Early exploration mainly focused on local structural oil reservoirs in the nose structural background on slopes, and the discovered oil reservoirs generally are distributed in point pattern or in some local areas. In the past few years, by deepening the study on hydrocarbon accumulation conditions in slope belts represented by Lixian slope and Wenan slope in Jizhong depression in the Bohai Bay Basin, we have reached some new findings, among them, three main cognitions are: (1) Lixian slope and Wenan slope are located in weak structural belts, both are structural-deposition slope type, with few local structures, which is favorable for forming stratigraphic-lithologic oil reservoirs[29]. (2) Hydrocarbon migration and accumulation were jointly controlled by both transporting capacity of faults and physical properties of reservoir sand bodies. Experiment results of hydrocarbon accumulation modeling indicate that when transporting capacity of faults is low, hydrocarbons migrate in several pathways, likely forming multiple oil-bearing layers in low positions (Fig. 4); when transporting capacity of faults is high, hydrocarbons migrate along predominant pathways with good physical properties to higher locations of the slopes in step style, thus oil-bearing layers only distribute in some local regions (Fig. 4). (3) We have established new modes of hydrocarbon accumulation in the above two types of weak structures. In Wenan slope, there is no oil source rock basically, but well-developed sand bodies, the hydrocarbons there came from the major oil generation trough of adjacent Baxian sag. Faults, sand bodies and unconformities make up the composite conducting pathways. Hydrocarbons mainly migrated to distal positions in step style. Accordingly, we have established several new modes of hydrocarbon accumulation (such as shallow structure-river channel sand in outer belt of the slopes, delta front sand bodies in inner belt of the slopes, etc.). In contrast, in Lixian slope, there develop a hydrocarbon generation layer (mainly the lower member of Es1), but the sand bodies are poorly developed, hydrocarbons there mainly migrate to proximal positions near source rocks, which is favorable for forming several types of oil reservoirs (such as faulted nose sand lens with low amplitude and updip pinch-out sand bodies, etc.). Guided by the above study, we have carried out overall evaluation on Lixian slope and Wenan slope, and newly found oil reservoirs have features of vertical superimposition (intervals) in vertical direction and continuous distribution on plane, forming two large-scale monolith reserve zones of 108 t order.
4. Key technologies and practices of re-exploration program In the implementation process of re-exploration program in oil-rich sags, the exploration targets have apparently changed
Fig. 4. Geologic model and physical modeling results of hydrocarbon migration in slope belt.
from structural plays to stratigraphic-lithologic plays, and from thick sand oil reservoirs to thin interbedded sand oil reservoirs. 4.1. Merged 3-D seismic prospecting technique in whole sags In the new round of overall study of oil-rich sags, we firstly evaluated and classified the quality of old 3-D seismic data, then repeated a new 3-D seismic acquisition in favorable hydrocarbon accumulation areas and carried out 3-D seismic acquisition over areas not covered in the previous survey; after that, we have conducted an overall merging of fine 3-D
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seismic processing[30]. In order to merge 3-D seismic data of different surveys, we adopted a unified definition of coordinate system and bin homogenization in the whole area. During the merging processing, we utilized some key techniques, such as establishment of near surface model, multi-domain fine pre-stack denoising, merged high-resolution velocity modeling, consistency processing of amplitude, phase and frequency, and moveout correction processing etc. In the overall merged processing of 3-D seismic data in oil-rich sags in the Jizhong depression in the Bohai Bay Basin, we developed pre-stack depth migration processing technique for mass data in super-large areas, established a super computer group (including 30 000 cores, with running speed of 1×1015 times/second), realized migration processing to 105 TB mass data (equal to 200 portable computers with 500 GB), and provided computation services with a high efficiency and high performance for overall merged 3-D seismic processing. By using the above techniques and methods, the differences in seismic data amplitude, phase and frequency between various surveys were eliminated, the merged 3-D seismic data set has a much better imaging for delineating faults. By adopting these techniques and methods, we have estab-
Fig. 5.
lished an overall merged 3-D seismic data set covering some oil-rich sags (Langgu, Baxian and Raoyang, etc.) in the Jizhong depression in the Bohai Bay Basin, with an area of 1×104 km2 (Fig. 3). This lays a solid data foundation for establishing high-resolution stratigraphic sequence frameworks, in-depth study of structures and fine delineation of sand bodies etc. in the re-exploration program of oil-rich sags. 4.2. Facies-based fine prediction technique for reservoirs As the major exploration targets in the re-exploration program of oil-rich sags are stratigraphic-lithologic traps, more sophisticated reservoir prediction techniques with seismic data are requited. Through years of study and practice, we have developed facies-based geological-seismic reservoir prediction technique (Fig. 5). Based on high-resolution 3-D seismic data, this technique involves establishing the structural-stratigraphic framework, classifying various types of depositional systems by combining geologic facies with seismic facies, dividing prediction into early stage without well/with few wells and high-accuracy stage with more wells, predicting lithologies, sand bodies, petrophysical properties and hydrocarbon bearing properties
Technical flow chart of facies-based geologic-seismic reservoir prediction.
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with the optimum geophysical methods selected, and finally, making comprehensive evaluation of all these prediction results to pick out reservoir prediction results coinciding with the reality to direct exploration deployment. In this technique, the constraints of the sequence model, structure model and deposition model are the keys for facies-based reservoir prediction, which is helpful for improving the accuracy of reservoir prediction; frequency spectrum decomposition, cooperative inversion of acoustic impedance and reservoir parameters, merging amplitude and frequency, etc. techniques can improve the accuracy of prediction of thin reservoirs. We have applied the above methods and techniques to finely delineate spatial distribution of reservoir beds in variable and complex sedimentary facies, which can improve drilling success. There developed fluvial channel sand bodies in the outer belt of the Wenan slope in Baxian sag, and fluvial channel sand bodies in delta front facies in Es3 in its inner belt. We utilized faciesbased fine prediction techniques for reservoir beds to describe spatial distribution of reservoir sand bodies there. Out of 39 wells deployed according to our study, 22 yielded industrial oil flows, with a success rate of exploratory wells of 56.4%, which is 16% higher than that in 2006, achieving an overall exploration breakthrough in inner and outer belts, in multiple layers and in multiple domains. 4.3.
Stimulation techniques for complex reservoirs
In the process of implementing re-exploration program in oil-rich sags, some new domains (such as oil reservoirs in (extra-) deep buried hills and intra-buried hills, oil reservoirs with low porosity and low permeability, tight oil reservoirs, etc.) have gradually become important exploration targets. Accordingly, some corresponding advanced and applicable exploration techniques (fracturing stimulation technique for carbonate reservoir beds in deep buried hills and sand reservoir beds with low porosity and low permeability, volume fracturing stimulation technique for tight oil, etc.) have been formulated or developed. By promotion and application, oil and gas production in individual wells has been greatly improved, and oil reservoir performance has been raised. The bottom-hole temperature in the Niudong buried hill oil reservoir in the Baxian sag is up to 201 C. Aiming at the problem of high temperature and too rapid acid-rock reaction (which is unfavorable for creating long fractures), we have developed the VES clean acid formula (acting as host acid in acid fracturing) with a high viscosity, super high temperature resistance and stable performance, and created a volume fracturing stimulation technique (prefixing exploring fractures, using main fractures for communication and auxiliary fractures for supplement, diversion of fracturing fluid, multiple stage injection, closed acidizing) to conduct large-scale acid fracturing with multiple stages, achieving network volume stimulation effect (main fractures act as high speed channels, auxiliary fracture nets act as supplement)[31]. After fracturing, the well
yielded a daily oil production of 642.91 m3, and daily gas production of 5.62×104 m3, marking it the deepest oil in carbonate buried hills with the highest temperature in eastern China. For the Lixian slope in the Raoyang sag, aiming at tricky issues such as fine lithology, low hardness, thin interbedded layers, etc., we have conducted analysis of oil reservoir performance, study of layer selection for fracturing and optimization of technique parameters, come up with a new fracturing strategy of rational scale, moderate sand ratio, anti-swelling in the whole process, moderate viscosity reducing[32]. The new fracturing strategy has been used for 11 well times, with a success ratio of 92%, resulting in oil production increase from 0.031.83 t/d to 4.2926.72 t/d. The effect is apparent, and oil reservoirs and reserves quality have been improved. 4.4.
Quick and efficient support drilling techniques
In an ultra-deep buried hill (Niudong 1 buried hill) at the western slope belt in the Baxian sag in the Jizhong depression in the Bohai Bay Basin, the technique combination of formate drilling fluid with strong inhibition and high temperature resistance, special drill bit, downhole pressure impulse generator, rotary blowout plug at the well head to control pressure, oil-in-water low-density drilling fluid was used to realize low-pressure underbalanced drilling at 6 000 m well depth. The technique combination resulted in an average rate of penetration (ROP) increase of 14.7%, average drilling footage of individual drill bits 6.3 times as much as previous record, fewer drill bits used and less tripping times, which shortened drilling cycles, reduced exploration cost, protected oil and gas layers, and achieved fast and safe drilling. Four drilling platforms in the Guan structure in the Langgu sag were set up to conduct multi-lateral and extended reach drilling. Among them, the Gu 43 platform has supported drilling of more than 10 exploratory wells, which made it possible to drill in urban areas, and effectively promoted exploration discoveries in the Langgu sag. 4.5.
Practice of re-exploration program
The Raoyang sag, which lies the central part of the Jizhong depression in the Bohai Bay Basin and covers an area of 5 280 km2, is taken as an example to illustrate. According to the results of a new round of hydrocarbon resources evaluatoin, initally in-place oil resources in the Raoyang sag are 11.7×108 t. As of 2005, its proven in-place oil reserves were 6.52×108 t, accounting for 68.2% of the total proven in-place oil reserves (9.56×108 t) in the Jizhong depression; its remaining in-place oil resources are 5.18×108 t, accounting for 34% of the total remaining in place oil resources in the Jizhong depression, clearly, it is an oil-rich sag with the biggest exploration potential in the Jizhong depression. Based on the abundant remaining oil and gas resources in the Raoyang sag, the re-exploration program with innovated exploration methods (Fig. 6) resulted in large-scale reserve additions in several blocks and multiple domains.
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Fig. 6. Flow chart of re-exploration program in oil-rich sags in rift basins.
4.5.1. sag
Setting up of 3-D seismic data base for the whole
Since 2006, based on analysis of old 3-D seismic data quality, we conducted an overall deployment and step by step new acquisition of 3-D seismic data over 11 blocks covering an area of 2 297 km2; launched massive merged 3-D seismic processing, and set up 3 415 km2 integrated 3-D seismic data volume. By eliminnating the differences in amplitude, frequency and phase among different 3-D seismic surveys, the massive merged 3-D seismic data has a clear composite wave features, accurate fault imaging and distinct fault structures, setting a fine 3-D seismic data base for conducting sequence stratigraphic analysis, structural study, local structure confirmation, and analysis of deposition systems and sand bodies in this sag. 4.5.2. Reconstruction of structural configuration and depositional history as well as reservoir architecture in the sag (1) Structure reconstruction. By 3-D structure interpretation in the whole sag, we realized the change from previous line interpretation in blocks to full 3-D interpretation in sags, then our understanding of structural belts and structural styles are more systematic and complete; especially, our understanding of structural features of buried hills are deepened. In the Changyangdian buried hill belt, our work changed from confirming traps on the buried hill top to finely confirming structures on the Wumishan Formation top, finding several new buried hill traps (Chang 3, Chang 6, etc.). In the Suning buried hill belt, we found the Ninggu 8 buried hill trap (with a local trap area of 6 km2), providing favorable exploration targets for buried hill exploration. (2) Deposition reconstruction. Based on merged 3-D seismic data, we carried out fine sequence stratigraphic division and correlation in the whole sag. After that, we examined the depositional systems of 10 third-order sequences and 30 systems tracts in the Ek-Es formations carefully, completing fine
industrial mapping of systems tracts for each sequence in the whole sag, and finely describing the distribution scopes of favorable sedimentary facies belts in various systems tracts. (3) Reservoir reconstruction. With an emphasis on deep (Es3-Es4) reservoir intervals, we rechecked the reservoir beds. The study results show deep clastic reservoirs are characterized by coexistence of primary and secondary pores. We confirmed a lower depth limit of predominant reservoir exploration (700 m deeper than the previous lower limit), greatly expanding exploration domain. Moreover, by overlapping four types of maps (sedimentary facies, diagenetic system, diagenetic stage and formation pressure), we picked out several favorable areas of deep reservoir beds, including the Ma 99 well block in the Maxi trough, the Ninggu 10 well block in the Hejian trough, the Lu 44 well block in the Liuxi trough, the Qiangshen 1 and Huang 3 well blocks in the Raonan trough, etc. 4.5.3. Quantitative characterization of the spatial distribution of oil and gas resources; establishment of new hydrocarbon accumulation models in various domains Based on fine classification and evaluation of source rocks with well logging data, we modeled hydrocarbon migration and accumulation processes in this sag, described hydrocarbon migration and accumulation processes in various geological periods and spatial distribution of hydrocarbon resources in various blocks and stratigraphic intervals (Fig. 7), and predicted the major distribution areas of the remaining hydrocarbon resources. On the basis of the above study, we made a comprehensive analysis of hydrocarbon accumulation conditions in various blocks and domains (such as trough region, slope belt and buried hill, etc.), and established hydrocarbon accumulation models in various domains in the Raoyang sag. In the Maxi-Liuxi troughs, we set up new models of sand oil reservoirs in delta front facies, and sand oil reservoirs in fluvialchannels by fault and sand coupling, etc. For the Lixian slope, we established a new reservoir formation model of low-amplitude nose structure sand lens and sand updip pinch-out. For subtle buried hills, we built three kinds of hydrocarbon accumulation models in buried hills: “old reservoir-old sealing coupling” (Chang 3), “red cap rock-lateral migration” (Ninggu 8x), and “big hill-crest accumulation” (Hu 8)[26,33]. These results played an important role in the overall investigation, trap discovery and confirmation, favorable target evaluation and selection of prospects in the above blocks and domains. Directed by the research results of the overall understanding and overall evaluation, with stratigraphic-lithologic oil reservoirs and subtle buried hill oil reservoirs as the major targets, we implemented an overall deployment and conducted the re-exploration program in steps in some favorable blocks (Lixian slope, Maxi-Hejian troughs, Changyangdian buried hill belt, Suning buried hill belt and Sunhu buried hill belt, etc.) in Raoyang sag, achieving important hydrocarbon
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Fig. 7.
Prediction of oil and gas distribution in major layers of Raoyang sag.
exploration results. The new 3P in place oil reserves are 2×108 t (5 727×104 t proven reserves), a total new production capacity of 114.5×104 t, realizing large-scale production and reserves increase. With the above methods, we implemented re-exploration program in some oil-rich sags (Baxian and Langgu, etc.), achieving major discoveries of in-place oil and gas reserves with a magnitude of 108 t. In the re-exploration program in the Baxian sag, we established composite oil reservoir model (including Es1-Ng low-amplitude structure-fluvial channel sand lithology) in the outer belt of the Wenan slope, lithologic oil reservoir model in delta front sand bodies controlled by structure-sedimentary slope break in the inner belt of the Wenan slope, and hydrocarbon accumulation model in extra-deep buried hill in the Niudong fault terrace in the steep slope belt in the western Raoyang sag. The re-exploration program is fruitful, bringing about new exploration discoveries of 1.15×108 t new 3P in-place oil reserves, and the discovery of the extra-deep and high temperature buried hill oil and gas reservoir in the Wumishan Formation in the Niudong 1 Field. The application of the above re-exploration methods in the exploration in the Aer sag (an oil-rich sag in the Erlian Basin) sped up its exploration pace. Since exploration drilling began in this sag in 2008, thick and high quality source rocks were found in the Well Aer-1, and high-productivity industrial oil flow was obtained in the Well Aer-3. By the year of 2010, a total of 10 469×104 t were added to the probable and possible in place oil and the discovery was named as the Aer Oilfield. It took only three years from drilling the first well to the discovery of the new oil field. Compared with similar sags, its exploration cycle is shortened by 5-10 years, achieving scientific, efficient and quick exploration of the new sag. Currently, with a 40×104 t annual production capacity, the Aer Oilfield has become an important oil and gas production base in the
Erlian Basin, contributing greatly to the steady increase of oil production in the basin[34]. The implementation of re-exploration program in oil-rich sags in the Jizhong depression in the Bohai Bay Basin and the Erlian Basin has brought about new breakthroughs and large discoveries in oil and gas exploration successively. Since 2009, the reserve addition has been kept at 108 t or so per year (Fig. 8), achieving positive results in oil and gas exploration, and blazing a new trail of deepening exploration in mature explored areas. Currently, the discovery maturities of oil resources in oil-rich sags (Raoyang sag, western Liaohe sag, Qikou sag, Dongying sag and Bozhong sag, etc.) in the Bohai Bay Basin are generally between 40%60%, representing a high exploration maturity. However, in the above oil-rich sags, the remaining in-place oil resources are up to 105×108 t, accounting for 78% of the remaining in-place oil resources in the Bohai Bay Basin, thus they have resources potentials for continuous and deepening exploration. In the Jiyang depression, through continuous and deepening investigation of hydrocarbon accumulation patterns in oil-rich sags, efficient exploration has been continued for 32 years since 1983, with a new proved in-place oil reserve of 1×108 t/year. This means that oil-rich sags in old explored areas are still important exploration regions for discovering large scale reserves.
5.
Conclusions
Oil-rich sags in the Bohai Bay Basin play an important role in oil and gas exploration and production in China. Still abundant in remaining oil and gas resources, they vary widely in exploration maturity, are the major targets for oil and gas production and reserves addition, thus, it is necessary to carry out re-exploration program in these sags. As a complex and systematic project, re-exploration program in oil-rich sags
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Fig. 8.
Total oil reserve (3P) growth in Jizhong depression in Bohai Bay Basin and Erlian Basin.
must be guided by new concepts on hydrocarbon accumulation, supported by advanced and applicable new exploration (engineering) techniques, to conduct the overall investigation, overall evaluation and overall arrangement. Oil and gas exploration practices show that implementation of the re-exploration program in oil-rich sags can realize large-scale new oil and gas reserve additions in mature explored areas. Moreover, the ideas and methods involved in the re-exploration also have practical guiding significance for oil and gas exploration in newly found oil-rich sags. Therefore, we should constantly renew our geologic concepts, positively promote technical progress, and carry out re-exploration in oil-rich sags vigorously, so as to promote new progress of oil and gas exploration in oil-rich sags in the Bohai Bay Basin.
[7]
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