Fracture-related diagenesis: a record of fluid flow through the Thamama Group, UAE
Al Blooshi, Mariam Nasser Abdulla
Most of the hydrocarbon reservoirs in the world are carbonates, and most of these carbonate reservoirs are fractured. Fractures can form due to tectonic activity, mainly associated with fold and faults, and as a result of diagenesis. In many cases, the fractures in carbonates are cemented due to fluid flow, where these fluids precipitate cements. The presence of fractures can enhance reservoir fluid flow if the fractures were open and connected. This thesis focuses on carbonate reservoir fractures, and understanding the evolution of fluids from the cements that have precipitated within them. This thesis relates regional tectonic events to the formation of fractures, and of the environment and temperature of the fluids of precipitation in cement fractures the Early Cretaceous Thamama Group, in the United Arab Emirates (UAE). First, samples were studied from the subsurface in a highly faulted oil Field (A) located in South East Abu Dhabi. Core samples were taken from wells close to major faults in the field in both oil and water legs. Second, fractures in outcrop in Wadi Rahaba, Ras Al Khaima (RAK) in the Northern Emirates were studied where the Upper Thamama Group is exposed. The field study in the outcrop included the fracture orientation and cement types. The fractures in the outcrop were divided into two main generations, F1, (en-echelon) and F2 and they were both fully or partially cemented. The main tectonic events that affected the fracture formation in the Thamama Group are foreland autochthon in the Precambrian to Lower Cretaceous, a frontal triangle zone in Neogene and Dibba zone which consists of Hawasina units (Triassic to Cretaceous), and the Sumeini units (Lower to Middle Cretaceous). F1 is related to the NS orientation fracture system consistent to the Arabian Trend sets caused by Cenozoic compression. F2 is related to the EW orientation fracture system matching Tethyan extensional trend sets. Petrographic analysis of the subsurface thin sections revealed the presence of three main sets of fractures. Fracture Set 1 (cemented), Fracture Set 2 (open) and Fracture Set 3 (cemented, only in Lower Thamama). The fracture cement included equant and blocky calcite as well as saddle dolomite. Cathodoluminescence (CL) analysis assigned the number of cement zones in each cement type in the fractures, and revealed more cement zones in the Lower Thamama reservoirs than the Upper. The most important diagenetic events were cementation and dissolution, which took place towards the end of the paragenetic sequence. The reservoirs contained significant amounts of stylolites, dissolution seams and bitumen, which were associated with most of the dissolution events. mMg/mCa obtained from in situ elemental analysis showed variation through the calcite and dolomite cement zones in the different reservoirs of the Thamama Group. This was inferred to be due to temperature changes. The Upper Thamama Reservoirs (A, B, and C) show lower mMg/mCa (0.072-0.48) than the Lower Thamama reservoirs (F,G) (0,4-1.3), meaning that the Upper Thamama fracture calcite cements were precipitated at higher overall temperatures than the Lower Thamama reservoirs. Mn-Fe analysis allowed an understanding of the redox index through the different cement zones, in both Fracture Set 1 and Fracture Set 3. Analysis of Sr showed the absence of exotic fluids role in the diagenetic system. In-situ (SIMS) δ18OVPDB values were obtained for the calcite cementation history of the two fracture sets in the five reservoirs of the Thamama Group. The δ18OVPDB analysis indicated that Fracture Set 1 has a longer cementation history than Fracture Set 3, and has wider range of temperatures (58-128°C). A comparison of the outcrop analysis results and the subsurface reservoir was established at the end to distinguish the similarities and differences between the subsurface and outcrop in fracture types, fracture cement types and characteristics of the elemental analysis curve behaviours. The fracture cement in both subsurface and outcrop seemed to be precipitated at deep burial environment.