Micro-geological causes and macro-geological controlling factors of low-resistivity oil layers in the Puao Oilfield

Petroleum Science, Jul 2009

Low-resistivity oil layers are often missed in logging interpretation because of their resistivity close to or below the resistivity of nearby water layers. Typical low-resistivity oil layers have been found in the past few years in the Putaohua reservoir of the Puao Oilfield in the south of the Daqing placanticline by detailed exploration. Based on a study of micro-geological causes of low-resistivity oil layers, the macro-geological controlling factors were analyzed through comprehensive research of regional depositional background, geological structure, and oil-water relations combined with core, water testing, well logging, and scanning electron microscopy data. The results showed that the formation and distribution of Putaohua low-resistivity oil layers in the Puao Oilfield were controlled by depositional environment, sedimentary facies, diagenesis, motive power of hydrocarbon accumulation, and acidity and alkalinity of reservoir liquid. The low-resistivity oil layers caused by high bound-water saturation were controlled by deposition and diagenesis, those caused by high free-water saturation were controlled by structural amplitude and motive power of hydrocarbon accumulation. Those caused by formation water with high salinity were controlled by the ancient saline water depositional environment and faulted structure and those caused by additional conductivity of shale were controlled by paleoclimate and acidity and alkalinity of reservoir liquid. Consideration of both micro-geological causes and macro-geological controlling factors is important in identifying low-resistivity oil layers.

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Micro-geological causes and macro-geological controlling factors of low-resistivity oil layers in the Puao Oilfield

Pet.Sci. Micro-geological causes and macro-geological controlling factors of low-resistivity oil layers in the Puao Oilfield Tan Fengqi 1 2 Li Hongqi 1 2 Liu Hongtao 0 Jiang Fucong 0 Yu Hongyan 1 2 0 The No.7 Oil Production Factory, Daqing Oilfield Company , PetroChina, Heilongjiang 163515 , China 1 State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum , Beijing 102249 , China 2 School of Resource and Information Technology, China University of Petroleum , Beijing 102249 , China Low-resistivity oil layers are often missed in logging interpretation because of their resistivity close to or below the resistivity of nearby water layers. Typical low-resistivity oil layers have been found in the past few years in the Putaohua reservoir of the Puao Oilfield in the south of the Daqing placanticline by detailed exploration. Based on a study of micro-geological causes of low-resistivity oil layers, the macro-geological controlling factors were analyzed through comprehensive research of regional depositional background, geological structure, and oil-water relations combined with core, water testing, well logging, and scanning electron microscopy data. The results showed that the formation and distribution of Putaohua low-resistivity oil layers in the Puao Oilfield were controlled by depositional environment, sedimentary facies, diagenesis, motive power of hydrocarbon accumulation, and acidity and alkalinity of reservoir liquid. The low-resistivity oil layers caused by high bound-water saturation were controlled by deposition and diagenesis, those caused by high free-water saturation were controlled by structural amplitude and motive power of hydrocarbon accumulation. Those caused by formation water with high salinity were controlled by the ancient saline water depositional environment and faulted structure and those caused by additional conductivity of shale were controlled by paleoclimate and acidity and alkalinity of reservoir liquid. Consideration of both micro-geological causes and macro-geological controlling factors is important in identifying low-resistivity oil layers. Daqing placanticline; Puao Oilfield; Putaohua oil layer; low-resistivity oil layers; microgeological causes; macro-geological controlling factors 1 Introduction In low-resistivity oil layers, the ratio of the resistivity of the oil layer to that of a formation water layer is less than 2 in the same oil-water system, that is to say, the resistivity index of the oil layer is less than 2 (Ouyang et al, 2005; Zhang et al, 2008) . Low-resistivity oil layers are often missed in petroleum exploration and development because of their complicated genesis, the limitation of vertical resolution of well logging instruments, and imperfect interpretation methods. At present, low-resistivity oil layers have attracted attention as one of the most promising targets for increasing reserves as well as secondary development in old oilfields (Mu et al, 2004). In China and other countries, the study on low-resistivity oil layers has acquired a great deal of achievements; for instance, people have formed a systemic understanding of micro-geological causes of low-resistivity oil layers (Zeng, 1991; Ouyang, 2002; Sun et al, 1998; Palar and Sutiyono, 1997; Worthington, 1997) . However, much remains to be done in the study of macro-geological controlling factors of low-resistivity oil layers and research into the matching relationship between micro-geological causes and macrogeological controlling factors has not been systematic. In this paper, various macro-geological controlling factors of the micro-geological causes of the Putaohua low-resistivity oil layers in the Punan and Aonan Oilfields, south of the Daqing placanticline, China were analyzed on the basis of core, well logging, geological and reservoir data. The results showed that a combination of micro-geological causes and macro-geological controlling factors was more important for understanding the genesis of low-resistivity oil layers. This can provide a geological basis for identifying low-resistivity oil layers effectively and adjusting development programs, and can also play an important role in exploring for zones of remaining oil. 2 Geological background of low-resistivity oil layers in Puao Oilfield The Puao Oilfield is located in the southern extension zone of the Putaohua structural belt, which is a third-grade resistivity oil layers with high bound-water saturation and high displacement pressure were formed in the Puao Oilfield. Pu220 Gu633 Gu684 The motive power of hydrocarbon accumulation determines the property and distribution of reservoir fluids when reservoir has the dual pore structure characteristics of micro-pores and flow pores. Water layers are formed when the motive power of hydrocarbon accumulation is less than the intergranular-pore capillary resistance of the reservoir. Low-resistivity oil layers are formed when the motive power of hydrocarbon accumulation is between the intergranularpore capillary resistance and the micro-pore capillary resistance and normal oil layers are formed when the motive power of hydrocarbon accumulation is greater than the micropore capillary resistance (Wu et al, 2006; Palar and Sutiyono, 1997; Li et al, 2006) . Therefore, low-resistivity oil layers caused by high bound-water saturation with the bound water in micro-pores and hydrocarbon accumulated in intergranular pores are formed in the reservoir with bimodal or multi-modal pore structure when the source rocks, migration pathway and the motive power of hydrocarbon accumulation are all appropriate. The diagenesis stage is a long process compared with the deposition stage and a series of complex physical and chemical changes reform the sandstone reservoir (Fu, 1998) . Compaction and cementation reduce and damage reservoir pores, and dissolution and metasomatism increase secondary pores of reservoir. The solution porosity in the Puao Oilfield is mainly from dissolving quartz and feldspar. Feldspar dissolution can generate secondary pores, and a large number of small secondary pores and big pores compose the dual pore structure of reservoir. Therefore, some oil layers become low-resistivity oil layers caused by high bound-water saturation interaction with the motive power of hydrocarbon accumulation. 3.2 High free-water saturation The main geological characteristics of low-resistivity oil layers caused by poor oil-water differentiation and wide oil-water transition zones are the development of primary intergranular flow pores, medium and grit sandstone lithology, high shale content, strong hydrophilic property of the rock, and low structural amplitude of oil layers. Because of the shallow buried depth of the reservoir, the pressure of source rocks and the formation overpressure are very low when oil and gas migrate from source rocks to the trap, and the motive power of hydrocarbon accumulation is mainly buoyancy. The difference between oil and water densities leads to high oil saturation (resistivity increase) at the structural high position and low oil saturation (resistivity decrease) at the structural margin, so low-resistivity oil layers caused by high freewater saturation at the structural margin are formed in the same reservoir system. Based on above analysis, the main geological controlling factors of such low-resistivity oil layers are the motive power of hydrocarbon accumulation and structural amplitude. The spatial distribution characteristics of developed lowresistivity oil layer wells were analyzed in the Puao Oilfield (Fig. 3). The results showed that the wells in which lowresistivity oil layers were observed were mainly distributed at the structural margin, and only a few were at the structural resistivity to form low-resistivity oil layers (Mu et al, 2004; Wei and Zou, 2005; Gao and Xie, 2006) . The cation exchange ability of clay minerals is usually measured by cation exchange capacity (CEC, mmol/100g), and the CEC values of common clay minerals are shown in Table 2. The clay minerals of the Puao Oilfield can be divided into two types according to their causes, one is clay minerals depositing with the terrigenous clast particles, and the other is authigenic clay minerals formed during late diagenesis. The type, composition and distribution pattern of terrigenous clast clay minerals are decided by ancient climate, provenance, and sedimentary environment, and the formation of authigenic clay minerals are mainly controlled by acidity and alkalinity of reservoir fluids. The type and content of clay minerals in the study area were analyzed by X diffraction (Fig. 4). The clay minerals are mainly illite, chlorite, and mixedlayer illite-montmorillonite with the average content of 51%, 29%, and 14% respectively, and they are all distributed stably in the study area. Kaolinite and mixed-layer chloritemontmorillonite are distributed in a few wells and there is no montmorillonite detected in samples from oil layers. 20 15 5 0 y c en 10 u q e r F Illite Chlorite Mixed-layer illitemontmorillonite Kaolinite Mixed-layer chloritemontmorillonite r 100 e y a l ixed it,%80 e ndm illrno 60 a o lilifo -omm te tn 40 tn ilte 20 te il n o C 0 0-20 The sedimentary environment of the Puao Oilfield changes from large-scale river delta front facies to shoreshallow lake facies. Under a warm and humid paleoclimate, some aluminum silicate minerals such as feldspar and mica The additional conductivity capacity of clay minerals is not only related to their types and contents but also to their distribution patterns, which are mainly bridge, filling, and cushion (Fig. 6), corresponding to “crumb”, “thin film”, and “patch” mud distribution patterns. The bridge distribution formed a high proportion of illite after weathering and diagenesis, and some aluminum silicate minerals formed kaolinite after weathering and leaching in an acidic medium. In the study area, because of the alkaline formation water, kaolinite is only distributed in a few wells and the content is low. Pyroxene, hornblende and biotite in terrigenous clasts can form montmorillonite after partial chemical weathering, but this is not found in the study area, because terrigenous clast clay minerals transform into authigenic clay minerals during late diagenesis. The types of clay minerals formed are closely related to the properties of formation water (salinity, ionic composition, acidity and alkalinity) (Fu, 2000; Zhao and Zhu, 2006) . The expanding clay mineral, montmorillonite, is formed at pH=6-8. Montmorillonite can transform into non-expanding clay minerals such as illite (pH=7-8) and chlorite (pH=8-9) with the increase of formation alkalinity (Guan et al, 2003; Zhao and Zhu, 2006) . In the study area, the water type is mainly high in NaHCO3 and the average value of pH is 8.2, so the formed montmorillonite from terrigenous clasts completely transforms into illite or mixedlayer illite-montmorillonite in this alkaline environment, and can transform into chlorite or mixed-layer chloritemontmorillonite in a stronger alkaline condition. Based on the above analysis, the types of clay minerals in the Puao Oilfield are mainly illite and mixed-layer illite-montmorillonite and low-resistivity oil layers can be formed because of strong additional conductivity of the two clay minerals. Fig. 5 is a cross plot of oil layer resistivity and the content of illite and mixed-layer illite-montmorillonite in the Puao Oilfield. It is shown that when the content of illite and mixed-layer illite-montmorillonite reaches above 80%, strong additional conductivity can decrease oil layer resistivity to develop lowresistivity oil layers. Low-resistivity oil layers Normal oil layers pattern leads to a continuous distribution of clay minerals in the reservoir rock to form a connected conductive network, and combined with high cation exchange capacity, lowresistivity oil layers caused by additional conductivity of clay minerals are formed. In the study area, the most common distribution pattern of clay minerals of such low-resistivity oil layer is bridge pattern. Although filling and cushion distribution patterns also have additional conductivity, continuous conductive networks are not formed and the conductivity capacity is too limited to influence oil layer resistivity, so it is difficult to form low-resistivity oil layers from these clay-mineral distribution patterns. SEM MAG:1.40KX Name:73550 4 Influencing factor of external causes of low-resistivity oil layers caused by thin interbeds of sand and shale The Puao Oilfield was located in the transition zone from delta front facies to shore-shallow lake facies, so hydrodynamic forces were relatively weak and thin interbeds of sand and shale were easily formed because of the reciprocating flow of water. In fact, the resistivity of a single layer of sandstone is not low, but due to the limitation of vertical resolution of well logging instruments, the actual measured value of resistivity decreases because of the influence of low-resistivity surrounding rocks. Thus, the resistivity difference of oilwater layers sharply decreases to form low-resistivity oil layers. Fig. 7 shows the logging interpretation results of Ao112 well in the study area. We can see that No. 59 and No. 63 layers are both low-resistivity oil layers. No. 59 layer is sandstone interbedded in mudstone. The resistivity well logging response of a single layer of sandstone is greatly influenced by the surrounding mudstone, so the response curve shows low-resistivity and peak characteristics. The testing results of No. 59 low-resistivity oil layer are 3.6 t/day of oil production and 0.2 t/day of water, which is classified as a commercial oil layer with low water cut. No. 63 layer is the mudstone interbedded in sandstone. The well logging response of a single layer of sandstone is greatly influenced by interbedded mudstone, so the measurement results are not a reflection of actual reservoir information, and the response curve shows low-resistivity and flat characteristics. The testing results of No. 63 low-resistivity oil layer are 5.31 t/day of oil production and no water, which is classified as a commercial oil layer. In the Puao Oilfield, sandstone interbeds, mudstone interbeds, and thin interbeds of sand and shale developed because of the special sedimentary environment, so lowresistivity oil layers of this type should be paid more attention in reservoir evaluation. 1 2 6 0 120 1.8 5 Conclusions 1) The formation and distribution of Putaohua lowresistivity oil layers in the Puao Oilfield are controlled by depositional environment, sedimentary facies, diagenesis, motive power of hydrocarbon accumulation, and acidity and alkalinity of reservoir liquid. 2) The low-resistivity oil layers caused by high boundwater saturation are controlled by deposition and diagenesis; those caused by high free-water saturation are controlled by structural amplitude and motive power of hydrocarbon accumulation; those caused by formation water with high salinity are controlled by ancient salty water depositional environment and faulted structure; those caused by the additional conductivity of shale are controlled by paleoclimate and acidity and alkalinity of reservoir liquid. 3) Special depositional environments lead to extensive development of thin interbeds of sand and shale in the study area. Because of the influence of low-resistivity surrounding rocks and limitation of the vertical resolution of well logging instruments, low-resistivity oil layers need to be paid special attention in the evaluation of oil layers. 4) The causes of low-resistivity oil layers are very complex and are influenced by multiple factors. Consideration of the combinations of internal and external factors, and micro-geological causes and macro-geological controlling factors can assist in revealing the nature of low-resistivity oil layers. Acknowledgements This work was supported by the National Natural Science Foundation Project (No.40173023). The authors would like to thank Wang Ming from the Research Institute of Exploration and Development of SINOPEC, and Zhang Shaohua from Changqing Oilfield Company for their helpful discussion and suggestions. Fu Q. Diagenesis effect on reservoir pores-taking the Rong-37 block of the lower Tertiary, Liaohe Basin as an example . 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Fengqi Tan, Hongqi Li, Hongtao Liu, Fucong Jiang, Hongyan Yu. Micro-geological causes and macro-geological controlling factors of low-resistivity oil layers in the Puao Oilfield, Petroleum Science, 2009, 246-253, DOI: 10.1007/s12182-009-0039-3