Fracture-related diagenesis: a record of fluid flow through the Thamama Group, UAE
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Date
29/11/2018Author
Al Blooshi, Mariam Nasser Abdulla
Metadata
Abstract
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.