GeoArabia, Vol. 9, No. 2, 2004 Gulf PetroLink, Bahrain Fault characterization, Thamama oil field, UAE Fault characterization by seismic attributes and geomechanics in a Thamama oil field, United Arab Emirates Yoshihiko Tamura, Futoshi Tsuneyama, Hitoshi Okamura and Keiichi Furuya ABSTRACT Faults and fractures were interpreted using attributes that were extracted from a 3-D seismic data set recorded over a Lower Cretaceous Thamama oil field in offshore Abu Dhabi, United Arab Emirates. The Thamama reservoir has good matrix porosity (frequently exceeding 20%), but poor permeability (averaging 15 mD). Because of the low permeability, faults and fractures play an important role in fluid movement in the reservoir. The combination of the similarity and dip attributes gave clear images of small-displacement fault geometry, and the orientation of subseismic faults and fractures. The study better defined faults and fractures and improved geomechanical interpretations, thus reducing the uncertainty in the preferred fluid-flow direction. Two fault systems were recognized: (1) the main NW-trending fault system with mapped fault-length often exceeding 5 km; and (2) a secondary NNE-trending system with shorter faults. The secondary system is parallel to the long axis of the elliptical domal structure of the field. Some of the main faults appear to be composed of en- echelon segments with displacement transfer between the overlapping normal faults (relay faults with relay ramps). The fault systems recognized from the seismic attributes were correlated with well data and core observations. About 13 percent of the fractures seen in cores are non-mineralized. The development of the fault systems was studied by means of clay modeling, computer simulation, and a regional tectonics review. The existing fluid-flow characteristics of individual faults and fractures in the field can be modeled using the present-day stress regime, with the maximum horizontal stress oriented north-northeast. Slip-tendency and dilation-tendency analyses simulating present-day regional stress conditions are indicators of fault and fracture transmissibility. The NNE-striking secondary fault system is parallel to the present- day maximum horizontal stress and could act as a flow conduit in the reservoir. INTRODUCTION Present-day stress regimes acting on fault and fracture systems can control where fluid flows within hydrocarbon reservoirs (e.g. Heffer and Dowokpor, 1990). The objective of this study is to accurately map the fault and fracture systems that affect fluid flow in the Thamama reservoir of an offshore United Arab Emirates oil field (Figure 1). Fault and fracture systems occur at many spatial scales, and are grouped here into: (1) seismic scale, (2) subseismic scale, and (3) micro scale. Seismic-scale faults are routinely imaged with 2-D and 3-D seismic data. Recent improvements, particularly in 3-D seismic acquisition and advanced processing techniques, provide clear images of subtle structural features and facies distribution in hydrocarbon reservoirs. By utilizing a variety of attributes extracted from 3-D seismic data (e.g. dip, similarity, curvature), the subsurface can be further analyzed to estimate the geometry of small-displacement faults (Figure 2). Subseismic-scale fault and fracture systems, however, are not possible to image using compressional waves alone. These more subtle systems may be indirectly modeled by using outcrop analogs, and physical experiments of both clay and sand materials. These analog models are then calibrated to the actual reservoir by using sensitivity analysis of the experimental settings through numerical simulation. This study used an analog-modeling apparatus consisting of two arms to apply regional stress, and a balloon for dome growth in the central part. These experiments were done at Southwest Research Institute in San Antonio, Texas, USA (D.A. Ferrill, D.W. Sims, A.P. Morris and D.J. Waiting, unpublished JNOC Report 2001). 63 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/9/2/63/4564557/tamura.pdf by guest on 25 September 2021 Tamura et al. Maydan Fateh 52°E54Salman ° Hamidiyah 56° Mahzam Bul Falah Fateh SW Hanine (Sassan) North Dome Sajaa Idd Al-Shargi Nasr Rashid Moveyeid Studied Field Juwaiza South Dome Kahaif Abu Al Bu Danah Dubai Mandous A-Structure North Bukhoosh Khubai Umm 25° Al-Karkara El Shaif Margham Bunduq Belbazem Arabian Gulf QATAR A-Structure South Bu Haseer Umm Umm Al Dholou 25°N Satah Al Salsal Bin Nashef Zakum Jarnain Umm Arzanah Saath Khusub Al Razbooth Mubarraz Al Dalkh Bu Jufair Umm C Abu Dalma Dhabi Al Anbar UNITED ARAB A EMIRATES Ghasha Zal Hair B Umm Al Lulu Dalmah Neewat Al Ghalan Hail Jarn Yaphour Shanayel Shuweihat Al Dabb'iya Rumaitha N Arjan 050 Ruwais Bida Al Qemzan Murban-Bab OMAN km Sahil Shams TURKEY Caspian SAUDI Sea Bu Hasa ARABIA SYRIA Asab Riqeah NDhafra Safah Med Sea IRAN 0 300 Marzuk Huwaila Haliba IRAQ km JORDAN Mushash KUWAIT 23° Arabian L-II-B Bu Qalla L1 G Lekhwair Madiq Gulf Al Barakah BAHRAIN Shah ° QATAR 2 23 Lekhwair E2 Daleel EGYPT Arabian Zarrarah Qusahwira Mender UAE Shield 1 OMAN 3 Mezoon Dhulaima 52°SAUDI ARABIA 54° Shaybah Red SUDAN Sea YEMEN Figure 1: Location map of the studied field in offshore United Arab ERITREA Arabian Sea Emirates. ETHIOPIA Gulf of Aden Micro-scale fractures are usually analyzed using WORK FLOW cores and Borehole Image Logs. In our study they were analyzed mainly from core samples (vertical Fault Geometry Interpretation and deviated wells), since the image quality of 1) Seismic attribute interpretation acquired Borehole Image logs did not allow for - For seismic-scale faults; conventional approach utilizing seismic attributes (dip, similarity, curvature) their detailed analysis. 2) Analog study - For subseismic-scale faults; modeling experiments, GEOLOGICAL SETTING numerical simulation and outcrop observations The oil producing Thamama Group (Figure 3) in the studied oil field consists of multiple layers of Integrated Geological Model chalky carbonate reservoirs with uniformly poor core permeability, averaging 15 mD, associated with relatively high porosity, frequently above Fault Flow Characterization 20 percent. The net-to-gross thickness is estimated 3) Geomechanics interpretation to be approximately 300/600 ft. The matrix - Slip tendency and dilation tendency properties of these reservoirs can be enhanced by 4) Well data interpretation - Fracture observation on core and borehole images a fluid conductive fault/fracture network, and by - Multi well interference tests fracture corridors. The domal structure of the field is likely to be related to a deep-seated salt diapir Figure 2: Work flow summary illustrating the of infra-Cambrian Hormuz Salt (Figure 4). integration of seismic mapped faults with the Previous studies provided evidence that wrench clay/sandbox analog modeling experiments and tectonics played an important role in the structural geomechanical interpretations. evolution in the region (Marzouk and El Sattar, 64 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/9/2/63/4564557/tamura.pdf by guest on 25 September 2021 Fault characterization, Thamama oil field, UAE UNITED ARAB EMIRATES HYDROCARBON HABITAT AND STRATIGRAPHY Seq. Central and Strati- Western Northeast Southeast Shows graphy (Sharland System Group et al., 2001) Limestone Simsima Fiqa Oolitic grainstone Upper Aruma Halul Cretaceous Laffan Dolomite Ruwaidha Tuwayil Mishrif Gypsum, anhydrite Shilaif Mishrif AP8 Sandstone Wasia Middle Mauddud Cretaceous Nahr Umr Shale Shu'aiba Bab Member Source rock Kharaib Aquifer Lekhwair Lower Lower Gas Thamama Cretaceous Habshan MFS J110 Manifa Oil Asab Hith Oolite 149 Ma Minor oil Arab ABC Arab-D (Lower) Arab-D Upper Hanifa MFS J70 Diyab Upper Jurassic Tuwaiq Mountain Lst. Hadriya Upper Araej Sila AP7 Lower Araej Uwainat Figure 3: Jurassic Izhara and Cretaceous stratigraphy of Jurassic Hamlah the United Arab Lower/Middle Marrat Emirates. 1995, Figure 4). At least two distinct tectonic events with different compressive stress directions are interpreted from the features of the Arabian Plate. One is the EW-directed ‘Oman stress’ associated with the Oman Mountain nappes and ophiolite obduction during the Mesozoic. The other is the NNE-directed Cenozoic ‘Zagros stress’. These different stress events reactivated pre-existing basement fault networks, and triggered salt swelling of the infra-Cambrian salt. The salt activation resulted in the structural growth of anticlines above the salt pillows and salt domes in the region. Rapid structural growth at the end of the Middle Cretaceous under the ‘Oman stress’ regime is one of the key factors for the structural doming and faulting of the studied field. FAULT GEOMETRY INTERPRETATION In order to investigate seismic-scale fault geometry, the following seismic attributes were extracted from single 3-D seismic data. (1) Time Dip; (2) Azimuth; (3) Edge Enhancement; (4) Instantaneous Phase; and (5) Curvature (Figure 5, Curvature not illustrated). In this particular oil field, the Time Dip-processed image gives the best visual fault enhancement. The faults were mapped using time slices in each attribute cube every 4 or 8 milliseconds. Lineations observed on time slices were interpreted as seismic-scale faults after being confirmed by other attributes and by vertical seismic cross-sections. Figure 6 shows an example of fault interpretation on a Time Dip attribute image. Two fault systems are present in this field. The main NW-trending normal faults are commonly