Regional diagenesis and its relation to facies change in the Upper Silurian, Lower Acacus Formation, Hamada (Ghadames) Basin, Northwestern Libya

Elfigih, Omar Bouzid (2000) Regional diagenesis and its relation to facies change in the Upper Silurian, Lower Acacus Formation, Hamada (Ghadames) Basin, Northwestern Libya. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Lower Acacus Formation consists of fourteen deltaic packages which change laterally across the Hamada Basin, NW Libya. Southern fluvial-channel sandstones prograded northward into coastal-deltaic sandstones and siltstones, and eventually to offshore-marine sandstones and shales. Each depositional environment in the Lower Acacus Formation produced a characteristic facies with specific compositional characteristics. -- Petrographic and petrophysical properties of selected sandstone units within the Lower Acacus Formation indicate that the sandstones have been altered by the diagenetic processes of compaction, authigenesis, and dissolution. Three diagenetic facies are identified from the southern basin flanks to the northern basin centre respectively: (a) quartz-cemented facies, (b) carbonate-cemented facies, and (c) clay-cemented facies. Each of the diagenetic facies contains a characteristic authigenic mineral suite reflecting a specific sequence of alterations which are (a) quartz-overgrowths and iron oxide, (b) carbonate cementation and dissolution, and (c) clay authigenesis respectively. -- Isotopic study of selected sandstone units reveals a diagenetic history involving meteoric fluids modified by increasing water/rock interactions and saline (connate) water mixing with burial. -- The major types of porosity have a regional distribution related to type of grain framework, matrix and cement in each depositional facies. Dissolution of carbonate cements and unstable grains was the dominant process to create secondary porosity; such porosity being associated with the proximal deltaic facies. Preserved primary porosity is mainly associated with the fluvial-channel facies. Microporosity and some fracture porosity are the dominant porosity types in the distal deltaic and reworked marine- offshore facies. With continued diagenesis, differential cementation and dissolution redistributed porosity and modified primary porosities in the Lower Acacus Formation. -- A multistage model of the leaching of carbonate cements and secondary porosity development is proposed. This includes: (I) a shallow depth (early) diagenetic stage wherein CO₂-charged meteoric waters flushed through channel systems of the fluvial sandstone facies in the southern part of the basin, and (2) a deeper (late) diagenetic stage mainly due to carbonic acid-rich waters expelled from maturing organic-rich Tanezzuft shales into the deltaic sandstone facies in the northern part of the basin. -- Early diagenesis recognized in the different sandstones of the Lower Acacus Formation has major impact on the present-day hydrologic system across the Hamada Basin. Present-day hydrologic zones have a distribution which is spatially expanded relative to the Lower Acacus diagenetic zones and indicates the role of fluid flushing and dilution through time. -- At the basin-wide scale the relationship between sandstone facies and the distribution of diagenetic elements is an important aspect of the Lower Acacus reservoir character heterogeneity. Recognized diagenetic facies heterogeneity includes: (1) iron-oxides and interstitial clay coatings associated with quartz-overgrowths in the quartz- cemented facies of fluvial origin, (2) calcite cement patterns in the carbonate-cemented facies of proximal deltaic origin, (3) ferroan-dolomite cement associated with the fine-grained, micaceous, clay-clast-rich sandstones of proximal deltaic origin, and with the reworked marine sandstone units, and (4) authigenic clay (kaolinite) and its effect on permeability throughout the various depositional facies. -- Integration of the many data sets results in a basin-wide 3-D geological model which includes four phases of major regional diagenetic changes: Phase I and II represent reduction of primary porosity and permeability in fluvial sandstone units. Phase III led to secondary pore formation and to modification of primary porosity in proximal delta front sandstone units. Phase IV represents pore-filling in distal delta front siltstones and reworked marine sandstone units. -- Improved regional basin analysis and computerized basin modelling must consider and apply the variability expected from a multitude of parameters ranging from primary depositional processes to processes of burial and subsequent modification of formation characteristics. Commonly-applied generalizations regarding diagenesis as determined from individual well studies are inadequate and in many instances perhaps invalid for developing productive regional model. Use of regional diagenetic modelling integrated with depositional facies and basin analysis represents an initial step in predicting regions of maximum enhanced and preserved porosity in the subsurface.

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