Identification, Origin and Distribution of Tarmats in Upper Zubair Sand Reservoir, Raudhatain Field, North Kuwait
- Shaikh Abdul Azim (Kuwait Oil Company) | Salah Al-Anzi (Kuwait Oil Company) | Yahya Ahmad Hassan (Kuwait Oil Company) | Stephen John James (BP Exploration) | Dipak Mandal (Kuwait Oil Company) | Hamad Al-Ajmi (Kuwait Oil Company)
- Document ID
- Society of Petroleum Engineers
- Abu Dhabi International Petroleum Exhibition and Conference, 5-8 November, Abu Dhabi, UAE
- Publication Date
- Document Type
- Conference Paper
- 2006. Society of Petroleum Engineers
- 1.2.3 Rock properties, 1.6.9 Coring, Fishing, 5.3.4 Reduction of Residual Oil Saturation, 5.1.3 Sedimentology, 4.3.3 Aspaltenes, 4.1.5 Processing Equipment, 3.3.1 Production Logging, 5.2.1 Phase Behavior and PVT Measurements, 1.6 Drilling Operations, 1.8 Formation Damage, 5.1.1 Exploration, Development, Structural Geology, 4.3.4 Scale, 2 Well Completion, 5.1.2 Faults and Fracture Characterisation, 5.6.1 Open hole/cased hole log analysis, 4.1.2 Separation and Treating, 5.2 Reservoir Fluid Dynamics, 2.4.3 Sand/Solids Control
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The Upper Zubair Formation is a giant reservoir consisting of 350 ft of excellent quality sandstone. This main producing reservoir has a 100 thick tar mat historically encountered near oil water contact. Recently, a well completed in the crestal part of structure above tar mat zone and in good quality sand did not contribute to oil production and presence of tar was suspected. A re-look at identification, origin and distribution of the tar mat has become critical for formulating future development strategy of the reservoir.
An appraisal well was drilled on the crest with a comprehensive data acquisition program including coring, fluid sampling and logging. Geochemical analyses of core plugs at every 4 ft were carried out along with fluid invasion studies. Observation from core material was compared with NMR logs to formulate criteria for identification of tar zones to supplement information from open hole and cased hole logs.
NMR transverse T2 relaxation histogram was observed to be affected by fluid properties and poresize distribution. There is a consistent shortened T2 distribution along tar mats resulting in missing porosity compared to neutron-density derived porosity. Geochemical analyses of core material indicates high asphaltene zones having undegraded paraffin and are isotopically lighter with immature biomarkers and higher concentration of aromatics. The observations indicate early origin of tarmats due to biodegradation of immature oil at a lower reservoir temperature. Later, undegraded oil with undegraded paraffin profile has migrated in to the reservoir after biodegradation process has ceased.
Biodegraded early origin of tarmat can not explain areal distribution over the field. Local compartmentalization of tar mat from light oil leaking along faults leaving behind heavier fractions during geological history is the possible origin for tar occurring above tar window. Early high Asphaltene content from immature charge, Asphaltene precipitation due to later light oil migrating to trap and gravity segregation explains the occurrence of tarmat in tar window near oil water contact. Early charges/asphaltenes were bio-degraded and incorporated in the tar zone.
Current study helps in identifying and mapping the tar and heavy oil zones in the reservoir. Distribution of tar mat of diverse origin needs to be understood well in advance to have realistic estimation of movable hydrocarbon.
Zubair reservoir is one of the prolific producing horizons in North Kuwait. It is overlain by three other producing reservoirs: Lower Burgan, Upper Burgan and Mauddud. The Upper Zubair sand horizon holds most of the inplace oil of the Zubair reservoir in estuarine channels. It is on continuous production since 1960.
Presence of tar was initially detected from produced oil in some of the wells completed in Upper Zubair sand and visual inspection of core materials. Geochemical studies were subsequently undertaken to demarcate occurrence of tar in all the Zubair reservoirs. Tar zone in Upper Zubair Sand was considered to be structurally controlled and occurred in a specific depth window around oil-water contact.
Later, presence of a thick zone of tar above the mapped window was detected from a crestal well completed in thick estuarine channels. Another well was drilled in the vicinity and full set of logging and coring followed by geochemical studies were carried out to understand the characteristics of this tar zone.
Tar zones are identified with certainty from geochemical study of core material as zones having high asphaltene content and high extract yield. In absence of core, indirect methods: such as PNL, PLT, openhole logs of later drilled wells and production history were used to demarcate the tar zones. The massive sands of Z44CH and Z46CH are the main locales of tar. Smaller channels of other layers are devoid of tar above the historical tarmat.
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