پديد آورندگان :
كدخدايي ايلخچي ، رحيم نويسنده دانشجوي دكتري گروه زمين‌شناسي دانشگاه فردوسي مشهد , , رضايي، محمدرضا نويسنده - , , موسوي حرمي ، رضا نويسنده استاد گروه زمين‌شناسي دانشگاه فردوسي مشهد , , كدخدايي ايلخچي ، علي نويسنده استاديار گروه زمين‌شناسي دانشگاه تبريز ,
چكيده لاتين :
Introduction:
An accurate and proper understanding of the hydrocarbon reservoir requires a comprehensive study of the sedimentary and diagenetic characteristics of reservoir rocks. Integration of the results from these studies with petrophysical data has an important role in identification of production zones and main factors controlling the reservoir quality. In this study, we demonstrated that rock typing of tight sands (as a type of unconventional reservoirs) by using a suite of well logs or electrofacies techniques is a good approach specially when integrated with flow properties can provide a real picture of reservoir heterogeneity related to dominant depositional and post-depositional processes controlling pore properties and resulted reservoir behavior.
Perth Basin is an N-S trending, long and narrow and asymmetrical rifting basin located in the southwest of Australia (Cadman et al. 1994). Whicher Range is one of the most important gas fields in the South Perth Basin. The sandstones of Willespie Formation, with Late Permian in age, constitute the main reservoir rock of the field (Crostella and Backhouse, 2000; Sharif, 2007). These sandstones are mostly reported as tight, with low porosity and low permeability.
Material and Methods:
In order to determine and differentiate different types of electrofacies in the reservoir, well log data from the Willespie Formation from four wells (WR1, WR2, WR3, and WR4) of the Whicher Range Field are used. Well log data used in this study are gamma ray, density, neutron, and sonic logs. In this research, cluster analysis technique based on cluster tree is used for grouping data and identifying reservoir electrofacies.
At the first step of the study, sedimentary rock types are identified based on integration of available core descriptions and petrographic studies, and clustering analysis of well log data for the gross reservoir interval comprising of sandy packages and shaly beds and interbeds. At the next step, petrophysical rock types are identified for the sandy packages based on integration of flow units and clustering analysis of porosity logs. Finally, the quality of sandy packages in the reservoir is interpreted in the framework of petrophysical rock types.
Discussion of Results:
Sedimentary rock types: In this section, reservoir from non-reservoir units was distinguished using clustering of well log data and verification of results with petrographic data. At this stage, three electrofacies were recognized:
EF1: Fine to coarse and very coarse grained clean sandstone with low shale volume and low GR value ( < 80).
EF2: Dirty sands, siltstones and very fine grained sandstones associated with some shale partings and clays and intermediate GR response ( < 130).
EF3: Shaly units include laminated carbonaceous-silty shale and argillaceous sandstones, very fine in size, with high shale volume and high GR response ( > 130).
Petrophysical rock typing of sandy packages: In the second step, petrophysical facies were identified for the tight sands and HFUs were classified.
Identification of hydraulic flow units: The porosity and permeability data were used to identify flow units of the reservoir. In this study, hydraulic flow units were calculated based on flow zone indicator (FZI( by using the method proposed by Amaefule et al. (1993). Five hydraulic flow units (A, B, C, D and E) were identified within the studied reservoir. Core description shows that HFU A and HFU B are related to the fine and very fine grained argillaceous sandstones and siltstones. Sandstones in HFU C vary from the fine grained to medium and coarse grained facies. HFU D and E are mostly related to the medium to coarse and very coarse grained sandstones.
Identification of electrofacies in sandy packages: In order to set a real and reasonable relation between HFUs and tight sands, well log responses of sandy packages were classified using a cluster analysis approach. At this stage, four electrofacies were generated from petrophysical logs (i.e. DT, RHOB and NPHI). We describe the general characteristics of petrophysical electrofacies and their connection with flow units and reservoir quality.
EF1 is mainly composed of medium to coarse and very coarse grained sandstones which is related to hydraulic flow units E and D. Fine to very fine and silty-argillaceous sandstones have a little share in this electrofacies and are related to HFU C and to some extent to HFU D.
EF2 shows a wide range of facies with respect to size varying from fine to coarse and very coarse grained sandstones. All flow units can cover EF2. Fine and very fine grained sandstones and siltstones are correlated with HFU A and B. HFU C is mostly related to fine to medium and occasionally coarse grained sandstones. Medium to coarse and very coarse grained-sandstones in this electrofacies coincide with HFU D and E.
EF3 includes low percent of facies in the reservoir. Fine grained sandstones in this electrofacies are related to HFUs A and B. In contrast, HFU C includes medium to coarse grained sandstones. Petrophysically, coarse grained facies in this electrofacies are close to those in EF4, and therefore they are discussed in one group (i.e. sand type 3).
HFUs in EF4 ranges from A to D. Fine grained and argillaceous sandstones and siltstones are correlated with HFU A and B. Medium to coarse grained sandstones are correlated with HFU D and a significant portion of HFU C.
In all EFs, low reservoir quality HFUs (HFUs A, B and some fraction of HFU C) can be correlated with the fine grained facies deposited in a low energy system. But, in the case of medium to coarse and very coarse grained sandstones, depending on the severity of the diagenetic processes, three sand types were recognized as follows:
a) Type 1 (very tight sandstones): This type coincides with EF1 and E and D flow unit are dominant. Medium to coarse and very coarse grained sandstones of this EF are severely affected by diagenetic processes (i.e. quartz, kaolinite and calcite cementation). As a result, reservoir quality in these sandstones has strongly been destroyed. Based on the core description, porosity is described very low. The average porosity and permeability in these facies are 3% and 0.5md, respectively.
b) Type 2 (tight sandstones): It is composed of medium to coarse and occasionally very coarse and granular sandstones that are related to HFU E and D and partially to HFU C. EF2 show a good agreement with type 2 tight sands. Diagenetic effects have also acted on these sandstones in the form of quartz overgrowths and kaolinite cementation. But in comparison to sandstones of type 1, pore spaces in this type, especially as intergranular, are relatively well preserved. Based on core description, porosity in this electrofacies is described as poor to fair. The average porosity and permeability are 6-7% and 0.87md, respectively. This type shows an intermediate reservoir quality between type 1 and 3.
c) Type 3 (sub-tight sandstones): It consists of medium to coarse and granular sandstones that are correlated with hydraulic flow unit C (dominant in EF3) along with HFU D and a significant portion of HFU C (dominant in EF4). In comparison with two others, type 3 sandy facies have the best reservoir quality. Similar to the type 2, porosity is intergranular and ranges from poor to fair based on core descriptions. Porosity and permeability are 12-13% and 1.65md in average, respectively.
Conclusion:
This study shows that investigation of reservoir facies from a suitable cluster of well logs (i.e. gamma ray, neutron, sonic and density) along with core porosity -permeability data is a practical way for identification and classification of reservoir rocks based on geological and petrophysical characteristics. On the other hand, studying the reservoir electrofacies in the framework of hydraulic flow units plays an important role in delineating of production zones.
EFs, were derived using five wells data from the Whicher Range Field, indicating the high heterogeneity of reservoir rocks varying from clean sands to shaly units. Detailed study of sandy packages in relation to petrophysical logs and their connection with HFUs resulted in distinction of three sand types (i.e. sand type 1, 2 and 3). These sandy units are basically tight, but based on severity of diagenetic effects, their porosity and permeability properties are different. The results of this study show that sand type 3 possesses the best reservoir quality distributed mostly in WR1 and WR4 wells in this field.
Keywords: Electrofacies, Hydraulic Flow Units, Reservoir Characterization, Tight Sands
