Distribution of petroleum enriched areas, Changling Sag, Southern Songliao Basin

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 36, Issue 4, August 2009 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2009, 36(4): 413–418.

RESEARCH PAPER

Distribution of petroleum enriched areas, Changling Sag, Southern Songliao Basin Li Ming1,*, Zhao Yimin1,2, Liu Xiao1,2, Zhang Yunmian1, Wang Yushan3 1. PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China; 2. China University of Geosciences, Beijing 100083, China; 3. PetroChina Exploration and Production Company, Beijing 100011, China

Abstract: Changling Sag of the southern Songliao Basin is a terrestrial hydrocarbon-rich depression, and according to the “sag-wide oil-bearing theory”, oil and gas can be found in its whole effective source rock area. But prolific oil and gas areas that are usually called “sweet spots” are still controlled by several main factors in distribution. Fault belts above low uplifts and basement faults not only are the pathways allowing for hydrocarbon migration, but can form the lateral barriers of reservoirs, they are the main boundaries of oil and gas enriched areas. On the other hand, the fracture zone developed in the vicinity of the faults has improved the reservoir connectivity and increased the chances to obtain high production. These make the fault-developed zone itself become a “sweet spots” belt rich in oil and gas that should be paid more attention in exploration. Key words: Changling Sag; prolific oil and gas area; controlling factors; fault-developed belt; sweet spots

1

Overview of study area

Changling Sag is located in the southern central depression area in Songliao Basin (Fig. 1). According to “Sag-wide Oil-bearing” Theory, Changling Sag is an oil and gas depression characterized by the hydrocarbon source rock with high TOC, large dimensions, moderate thermal evolution, great hydrocarbon generation and rich accumulation[1]. In recent years, the exploration practices in Jilin exploratory area indicate that there is distribution of oil and gas reservoirs in the sloped area beyond the second-order structure zone and the deep position of the depression, which presents the “sag-wide oil-bearing” trend. The “sag-wide oil-bearing”, however, does not mean that all the oil and gas in the entire sag can be produced commercially. Based on the sag-wide exploration, the focus is placed on finding the oil and gas enriched areas, i.e. “sweet spots”, which is still the dominant orientation in the current oil and gas exploration task[2-4]. The oil and gas enrichment and spatial distribution are largely controlled by the fault-developed belt formed during the depression period in Changling areas, and for this reason, finding the fault-developed belt and structural fracture-developed area is crucial to ensure high production of single wells.

2

Genesis and types of faults

Songliao Basin has mainly undergone three phases of major structural evolution: fault-depression, depression and structural reversion[5]. During the fault-depression period, i.e. Early Cretaceous, a group of growth fault systems with the dominant NNE orientation were formed due to the detachment of crustal bathyderm and regional stress field, largely distributed in the deep formation and basal surface of the basin. The generation of those faults has resulted in obvious control of formation, evolution and oil and gas distribution of the fault basin. In the early Late Cretaceous (100-85 Ma from now), the basin came into the depression phase and its sedimentary formation mainly included the upper part of Quantou Formation, Qingshankou Formation, Yaojia Formation and Nenjiang Formation. At that time, Changling Sag was controlled by extension and the faults formed in this period could be divided into two types according to the genesis: the first type is the inherited fault, the growth faults in the fault period came into play again with further deeper fault throw and greater scale, such as Da’an Fault, Honggang Fault, Anguang Fault and Taobao Fault; the second type is the interlaminar slip fault. There were more faults with small scale which primarily appeared above the T3 interface(top Denglouku Formation) and most of which were concentrated in Quantou Formation and Qingshankou Formation, with the significant reduction of faults when coming into Yaojia

Received date: 29 May 2008; Revised date: 21 May 2009. * Corresponding author. E-mail: [email protected] Copyright © 2009, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

Li Ming et al. / Petroleum Exploration and Development, 2009, 36(4): 413–418

Fig. 1

Geographical location of Changling Sag

Fig. 2 Seismic reflection profile in some area of Changling Sag

Formation upwards (Fig. 2). After the sedimentation of Nenjiang Formation, the entire basin came into the structural reversion phase and extended to the late Neogene. In this period, the normal faults formed at that time were subject to varying degrees of reconstruction due to compression from NW-SE direction, and reversions occurred especially in some growth normal faults and resulted in the reversion faults normal at bottom and abnormal at top while forming a series of reversion structural belts controlled by the reversion faults, which became a vital place for oil and gas accumulation.

3

Genesis and types of fractures

Observing a wealth of drilling coring data and comparing the shapes, distribution ranges and regularities of the fractures, we think that most of the developed fractures in Quantou Formation and Qingshankou Formation of Changling Sag are structural fractures with the extensional fracture surfaces relatively coarse and the flexural and shear fractures relatively straightened. It is believed through analysis that those fractures were formed basically in the late sedimentation of

Li Ming et al. / Petroleum Exploration and Development, 2009, 36(4): 413–418

Fig. 3 Variable dip coherent analysis at the top boundary in Quantou Formation in Daqingzijing area

Nenjiang Formation and before sedimentation of Da’an Formation[6]. The main reservoir in Changling Sag had been in diagenesis and rigid status. Under the effect of intensive compressional pressure (the maximum principal stress direction is 85-105º SEE) from the basin NW-SE direction, two groups of near-vertical conjugate shear fractures (the direction is 35-55º NE and 125-145º SE, respectively) and one group of transverse extensional fractures in parallel with the maximum principal stress direction were formed[7]. This category of fractures are readily generated above the basement faults at the structural axis while the faults in these places are well developed; in contrast, there are less developed structural fractures in the structural flanks and on the internal slope inside the depression. Fig. 3 is the variable dip coherent plane graph at the top boundary in Quantou Formation in Daqingzijing Oilfield, in which the blue black linear parts display the distribution characteristics of the faults and the blue black flaky parts that surround the faults reflect the fracture development, revealing that the parts surrounding the fault development belt are the structural fracture-developed areas as well.

4

Distribution of fault and hydrocarbon

The sedimentary formation in the depression period is the major oil and gas exploration strata in Changling Sag. The fault patterns in the depression period have exerted an important effect on the hydrocarbon distribution in the upper part of Quantou Formation, Qingshankou Formation, Yaojia Formation and Nenjiang Formation. Hence, it is of far-reaching importance for understanding hydrocarbon accumulation and reservoir through analysis of the fault characteristics in the period. On one hand, fault can serve as a

significant migration pathway, and on the other hand, it can serve as a lateral barrier in the hydrocarbon trap. In the meantime, the developed structural fractures in the vicinity of the faults also greatly improved the reservoir connectivity and enabled the high-producing chances to be enormously increased. Therefore, the fault-developed belts take the apparent control of the formation of oil and gas accumulation areas[8]. Fig. 4 reflects the planar features of fault distribution in the sedimentation period in Quantou Formation- Qingshankou Formation. It can be observed from the figure that there is a great regularity in the fault distribution. It can be mainly divided into two types: the first type is located in the formation above the basement faults that made further extension and tension, which resulted in faulting in the upper formation to form numerous, small-scale, dense fault belts, normally extending 2-4 km (maximum 6-8 km). Three large NNE fault-developed belts were obviously formed in Changling Sag and they are the west fault belt (West Dabushu-West Haituozi-Honggang fault belt), the central fault belt (Dabushu-West Huaaopao-East Haituozi-Da’an fault belt) and the east fault belt (East Daqingzi-Qianan fault belt), respectively, in which there is a great scale development in the west fault belt and the central fault belt, both being a long and narrow strip graben. The faults in the rifts are spread en echelon with their direction mainly NNW, some is NNE and near SN. In Haituozi area, a branch appears in the central fault belt and its strike direction is changed from NNE to NNW, converging with the west fault belt and disappearing. The other branch remains NNE direction extending to Da’an area, which still remains a graben shape overall. The fault horsts

Li Ming et al. / Petroleum Exploration and Development, 2009, 36(4): 413–418

Fig. 4 Fault distribution at the top boundary in Quantou Formation of Changling Sag

and taphrogens are formed among three fault belts and alternately arrayed[9]. In the south, there are fewer faults to the west of the central fault belts, generally the eastern dip slope with increased steepness. There are also fewer faults to the east of the central fault belt, which was a gentle syncline and gradually transformed into the western dip slope. The oil and gas accumulation is restricted by effective hydrocarbon source rock distribution, accumulated in the south of the east fault belt, the north of the central fault belt and the north of the east fault belt and developing the structural and structural-

lithological oil reservoir. The material source in the western dip slope area to the east of the east fault belt mainly originated southwestward from the sand body that presented updip pinchout, forming a great quantity of fault-lithological and lithological pools and a large-scale oil-bearing plane. There is the smooth syncline between the central fault belt and the east fault belt and gradually transforming into the western dip slope with less developed structural trap. The west branch of Baoqian delta is the contributor to the main reservoir sand body in this area with developed sand-shale

Li Ming et al. / Petroleum Exploration and Development, 2009, 36(4): 413–418

interbedded layers favorable to form lithology-related reservoirs. But based on the analysis of the trend of the thinning sand body and the combination of slope dips, the southern sand body is in small-angle skew intersection with the western dip slope, and therefore the lateral barrier condition is unfavorable with the trap difficult to form. The formation of a large-scale trap is largely dependent on the lateral barriers to be formed only by the effective combination of faults in the vicinity of the fault belts. The single sand layer thickness in the area is usually smaller than 5 m with the fault throw 10-20 m. The fault can effectively disconnect the thin sand-shale interbedded layers to form the fault-lithology reservoir. Hence, the fault-developed belts that are located above the basement rift and in the vicinity of them are one of the main oil and gas accumulation areas. The second type of faults is mainly located above the basement uplifts. As the basement uplift is usually located between two depressions, there are a greater faulting intensity in the basement rift in the depressions and stronger downward traction. Affected by adjusting equalization difference in the overlying formations, the interlaminar slip faults are usually formed in the formation above the basement uplifts, which shape the graben rifts. A low basement uplift-low central uplift was developed in Daqingzijing area (Fig. 4), in the southern part of which is Heidimiao sub-depression and the northern part of which is the north sub-depression, thus forming a near NWW structural saddle which the central fault belt and the east fault belt cross. During the depression period, the upper formation of the low central uplifts, by tensile stresses, formed the tensional faults in line with the extension direction of rifts during the fault depression period. During the structural reversion period, influenced by NW-SE compressional stress, two groups of faults were formed in the upper formation of the low central uplifts: one group are the NW tension torsional strike-slip faults in near parallel with the principal stress direction and the other group are the SE pressing torsional strike-slip faults perpendicular to the principal stress direction, in which the NW tension torsional strike-slip faults are relatively developed. Compared with the first type of faults, the second type of faults has low concentration and indefinite distribution direction. Hence, the fault distribution area in the upper formation of the basement uplifts is one of the main oil and gas enrichment areas.

5

Prediction of oil and gas enrichment belts

Changling Sag in the southern Songliao Basin is a NNE large syncline with sand body mainly from the southwest Baokang water system and the north water systems, abundant oil sources and developed reservoirs. The sand body is interbedded between the hydrocarbon source rocks, and belongs to the self-generation and self-preservation accumulation with rich hydrocarbon resources, widelydeveloped structures and fault-lithology oil reservoirs. Varied hydrocarbon distribution characteristics result from the different reservoir forming conditions of structures, faults,

transport layers, traps, etc[10]. There are better physical properties in Qingshankou Formation in the Sag with developed faults, which provide the prerequisite for oil and gas enrichment. It can be observed from the reservoir development characteristics that during the sedimentation period in Qing 1 Member to Qing 3 Member, the delta sand body gradually penetrated into the lake region with the facies zones from the delta-front facies to the pro-delta facies, which provided a reservoir space for oil and gas accumulation as well as created a barrier condition for the formation of lithological pools. There are large-scale, connected oil zones of Gaotaizi oil layer in the area and the whole Baoqian sand body is almost entirely oil-bearing and the major horizon for exploration. The distribution of Putaohua and Fuyu reservoirs above and below the sand body is controlled by faults and reservoirs due to their oil and gas from the hydrocarbon source rock in Qingshankou Formation. As the fault is the main channel for oil and gas migration, multi-layer oil and gas enrichment areas are usually located at the place where faults concentrate (Fig. 5). On the other hand, in the Sag, the shallow formation is smooth and lacks large structures and many small structures with lower amplitude were developed in two flanks of the syncline, which is unfavorable for forming large-structure oil reservoirs. The fault-barrier reservoir is formed only depending on the fault barriers. For example, the east fault belt crosses Daqingzijing Oilfield which is located in the south of Changling Sag and just at the axis of the Sag (Fig. 4). The attitude of stratum is the east dipping to the west of the fault belt and the material source came from Baokang sedimentary system. In Daqingzijing area, underwater distributary channels, river mouth bars and sheet sands were mainly developed. The fault extension direction is mainly NNW, nearly perpendicular to the material source direction, which can build the barrier in the updip direction of the sand body; therefore, a fault-barrier reservoir can be formed in the vicinity of the fault belt. To the east of the fault belt, the attitude of stratum is the west dipping and as there is updip pinchout in the reservoir, a lithological updip pinchout reservoir can be readily formed. A larger scale fault-lithology oil reservoir can be readily formed in the vicinity of the fault belt, for this reason, at slopes and smooth formation areas; the place in the vicinity of faults facilitates oil and gas accumulation and becomes reservoir. In Huazijing terrace located in the eastern slope of Changling Sag, oil and gas migrated upward along the slope. On the one hand, the developed faults are so favorable to oil and gas migration that Fuyang and Gaotaizi constitute the major oil layers; on the other Hand, Fuyang oil layer and Gaotaizi oil layer are joined or nearly joined on the both sides of the faults in the vicinity of which the oil and gas was accumulated to form the multi-layer fault blocks or fault-lithology hydrocarbon reservoirs. Haituozi-Huaaopao area located on the northwestern slope in Changling Sag is the southern extension part of Da’an thrust fault with the unit form as a reversion structure. The

Li Ming et al. / Petroleum Exploration and Development, 2009, 36(4): 413–418

Fig. 5 Reservoir profile in Well Hong 143, Well Hong 66, Well Da 45, Well Da 44 and Well Da 209

western fault belt and central fault belt cross the area where the faults were well developed with the sand body mainly from the west and southwestern material sources. The oil-bearing layers mainly include Fuyu layer, Gaotaizi layer and upper Sartu layer. Fuyu reservoir is controlled by the dark mudstone distribution limit in Qing 1 Member with its thickness greater than 50 m, which forms the lithological pool in a structural setting. As there is a high possibility that oil reservoir should be formed in the vicinity of densely distributed area of faults, the high-rate oil flow can be obtained in Fuyu layer by well testing, such as Well Hai 49, Well Hai 51 and Well Qian 152. The forming characteristics in Gaotaizi layer are different from Fuyu layer and closely related with the ratio of the total sand thickness to formation thickness in Qing 1 Member formation: if the ratio is greater than 40%, it mainly developed the structural reservoir; if the ratio is within 20-40%, it is mainly developed fault-barrier structure-lithology reservoir; if the ratio is smaller than 20%, it mainly developed sand lense-lithology reservoir. Well Hai 38 is located at the converging point between the central fault belt and the western fault belt with sandstone and faults well developed, in which the high-rate commercial gas flow was obtained in a structural trap setting. As the favorable placement area of fault-sand body was found in the structural low by means of seismic data, Well Hai 33 was deployed to result in producing high-rate flow in Qing 1 Member. Fig. 5 shows the reservoir profile across the western fault belt and central fault belt from east to west. Well Hong 143 is located in Honggang area in the western fault belt, in which the high-rate oil flow was obtained in Fuyu and Gaotaizi oil layers; Well Da 209 is located in Da'an area in the central fault belt, in which the high-rate oil flow was obtained in Fuyu and Yangdachengzi oil layers. Whereas there was less gas show in Well Hong 66 in the underdeveloped fault area of the central part and significant reduction in the oil production in Well Da 44. But, since there are fault barriers in the updip direction in Well Da 45, the fault-lithology trap is formed, and although its

altitude is lower than Well Hong 66, its oil production is higher than Well Hong 66, which proves that the faults in the structure low is the important controlling factors for oil and gas accumulation. On the other hand, it can be seen from Fig. 4 that the proven 3P reserves are mostly distributed in the vicinity of the fault-developed belt in the sag, which also accounts for the fault distribution controlling oil and gas enrichment to a greater degree.

6

Conclusions

There is a sag-wide oil-bearing trend in Changling Sag in Songliao Basin as a prolific oil and gas sag. But it should be realized that not all of hydrocarbon accumulations can be commercially produced in the prolific oil and gas sag. According to the characteristics of the middle-shallow geological structure and sedimentary evolution in South Songliao Basin, there are the conditions of forming large-scale lithological oil reservoirs within the delta-front facies zone near hydrocarbon-generating sag. But for the reservoirs of the close consistency between the sand updip direction and source direction, whether there is a barrier in updip direction becomes a controlling factor of forming of reservoir. A great number of developed faults and the fault-developed belt with regular distribution in Changling Sag have established the favorable conditions for reservoir forming of this type, in the vicinity of which a lot of hydrocarbon reservoirs in the structural trap are found while more and more fault-lithology reservoirs, controlled by the faults, are verified. The practices indicate that during the past exploration, the further study, discovery and verification of structure-related lithological oil reservoirs were made after identifying them. Along with the change of exploration concept and technological advances, a structure is determined in some exploration area with an eye open to the lithological trap at the same time. The fault-lithology trap, formed by the fault trap and its placement with sand body, has become the "window" of discovering a lithological reservoir in exploration. The study of formation,

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evolution and distribution of a fault-developed belt is one of the ways for seeking "sweet spots" of exploration.

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