L. F. De Ros

The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy

Diagenesis exerts a strong control on the quality and heterogeneity of most clastic reservoirs. Variations in the distribution of diagenetic alterations usually accentuate the variations in depositional porosity and permeability. Linking the types and distribution of diagenetic processes to the depositional facies and sequence-stratigraphic framework of clastic successions provides a powerful tool to predict the distribution of diagenetic alterations controlling quality and heterogeneity. The heterogeneity patterns of sandstone reservoirs, which determine the volumes, flow rates, and recovery of hydrocarbons, are controlled by geometry and internal structures of sand bodies, grain size, sorting, degree of bioturbation, provenance, and by the types, volumes, and distribution of diagenetic alterations. Variations in the pathways of diagenetic evolution are linked to (1) depositional facies, hence pore-water chemistry, depositional porosity and permeability, types and amounts of intrabasinal grains, and extent of bioturbation; (2) detrital sand composition; (3) rate of deposition (controlling residence time of sediments at specific near-surface, geochemical conditions); and (4) burial thermal history of the basin. The amounts and types of intrabasinal grains are also controlled by changes in the relative sea level and, therefore, can be predicted in the context of sequence stratigraphy, particularly in paralic and shallow marine environments. Changes in the relative sea level exert significant control on the types and extent of near-surface shallow burial diagenetic alterations, which in turn influence the pathways of burial diagenetic and reservoir quality evolution of clastic reservoirs. Carbonate cementation is more extensive in transgressive systems tract (TST) sandstones, particularly below parasequence boundaries, transgressive surface , and maximum flooding surface because of the abundance of carbonate bioclasts and organic matter, bioturbation, and prolonged residence time of the sediments at and immediately below the sea floor caused by low sedimentation rates, which also enhance the formation of glaucony. Eogenetic grain-coating berthierine, odinite, and smectite, formed mostly in TST and early highstand systems tract deltaic and estuarine sandstones, are transformed into ferrous chlorite during mesodiagenesis, helping preserve reservoir quality through the inhibition of quartz cementation. The infiltration of grain-coating smectitic clays is more extensive in braided than in meandering fluvial sandstones, forming flow barriers in braided amalgamated reservoirs, and may either help preserve porosity during burial because of quartz overgrowth inhibition or reduce it by enhancing intergranular pressure dissolution. Diagenetic modifications along sequence boundaries are characterized by considerable dissolution and kaolinization of feldspars, micas, and mud intraclasts under wet and warm climates, whereas a semiarid climate may lead to the formation of calcrete dolocrete cemented layers. Turbidite sandstones are typically cemented by carbonate along the contacts with interbedded mudrocks or carbonate mudstones and marls, as well as along layers of concentration of carbonate bioclasts and intraclasts. Commonly, hybrid carbonate turbidite arenites are pervasively cemented. Proximal, massive turbidites normally show only scattered spherical or ovoid carbonate concretions. Improved geologic models based on the connections among diagenesis, depositional facies, and sequence-stratigraphic surfaces and intervals may not only contribute to optimized production through the design of appropriate simulation models for improved or enhanced oil recovery strategies, as well as for CO2 geologic sequestration, but also support more effective hydrocarbon exploration through reservoir quality prediction.

Dolomite Diagenesis and Porosity Preservation in Lithic Reservoirs: Carmopolis Member, Sergipe-Alagoas Basin, Northeastern Brazil

The lithic sandstones and conglomerates of the Carmopolis Member of the Muribeca Formation (Aptian) were deposited bp fan deltas, alluvial fans, and braid deltas that prograded from low-grade metamorphic terrains into the Sergipe-Alagoas rift basin during

Multiple authigenesis of K-feldspar in sandstones; evidence from the Cretaceous Areado Formation, Sao Francisco Basin, central Brazil

ABSTRACT Abundant authigenic K-feldspar is present in continental sandstones of the Lower Cretaceous Areado Formation in Sao Francisco Basin, central Brazil. Feldspar authigenesis occurred in two major episodes. (1) In the Early Cretaceous K-feldspar precipitated as overgrowths, healing of microfractures, and replacement of detrital K-feldspar and plagioclase at very shallow depths. Authigenesis was related to meteoric dissolution of detrital volcanic feldspars, volcanic vitric rock fragments, and micas in a hot semiarid climate. (2) In the Late Cretaceous, while still at shallow depths, K-feldspar precipitated as abundant fine crystals associated with opal-CT, hematite, and mixed-layer illite/smectite. The feldspar has extensively filled the intergranular and, to a lesser extent, intragranular pores, and replaced mud intraclasts and volcanic rock fragments. The authigenesis in this second episode was related to percolation of meteoric waters through the overlying Upper Cretaceous alkaline volcanic rocks of the Mata da Corda Formation, as indicated by the δ18OSMOW of K-feldspars from 19.4 to 21.8‰.

Diagenesis of siliciclastic and volcaniclastic sediments in the Cretaceous and Miocene sequences of the NW African margin (DSDP Leg 47A, Site 397)

Abstract The Lower Cretaceous and Miocene sequences of the NW African passive continental margin consist of siliciclastic, volcaniclastic and hybrid sediments. These sediments contain a variety of diagenetic carbonates associated with zeolites, smectite clays and pyrite, reflecting the detrital mineralogical composition and conditions which prevailed during opening of the North Atlantic. In the Lower Cretaceous siliciclastic sediments, siderite ( −6‰ to +0.7‰ δ 18 O PDB , −19.6‰ to +0.6‰ δ 13 C PDB ) was precipitated as thin layers and nodules from modified marine porewaters with input of dissolved carbon from the alteration of organic matter. Microcrystalline dolomite layers, lenses, nodules and disseminated crystals ( −3.0‰ to +2.5‰ δ 18 O PDB , −7.2‰ to +4.9‰ δ 13 C PDB ) predominate in slump and debris-flow deposits within the Lower Miocene sequence. During the opening of the Atlantic, volcanic activity in the Canary Islands area resulted in input of volcaniclastic sediments to the Middle and Upper Miocene sequences. Calcite is the dominant diagenetic carbonate in the siliciclastic-bioclastic-volcaniclastic hybrid and in the volcaniclastic sediments, which commonly contain pore-rimming smectite. Diagenetic calcite ( −22‰ to +1.6‰ δ 18 O PDB , −35.7‰ to +0.8‰ δ 13 CPDB ) was precipitated due to the interaction of volcaniclastic and bioclastic grains with marine porewaters. Phillipsite is confined to the alteration of volcaniclastic sediments, whereas clinoptilolite is widely disseminated, occurring essentially within foraminiferal chambers, and formed due to the dissolution of biogenic silica.

Deposition, diagenesis and reservoir potential of non-carbonate sedimentary rocks from the rift section of Campos Basin, Brazil

A new petrographical study was performed on the rift section of the Lagoa Feia Group, Lower Cretaceous of the Campos Basin, eastern Brazilian margin. The primary constituents of the analysed rocks are siliciclastic and volcaniclastic grains, stevensite ooids and peloids, and bioclasts of bivalves and ostracods. This study focused on the clastic, stevensitic and hybrid rocks, as previous studies were limited to the bioclastic rudstones and grainstones that constitute the producing reservoirs. The rift sedimentation was mostly intrabasinal, with extrabasinal contribution concentrated close to half-graben border faults. The mixture of rounded volcanic fragments with angular quartz, feldspars and plutonic fragments in the sandstones and conglomerates indicates recycling of early rift epiclastic deposits, combined with first-cycle sediments eroded from uplifted plutonic basement blocks. Stevensitic ooids and peloids, formed in shallow, alkaline lacustrine environments, were mixed throughout the rift section with bivalve and ostracod bioclasts, and with the clastic sediments. Gravitational redeposition was promoted by intense and recurrent tectonism along the rift margins. The main diagenetic processes in clastic sandstones and conglomerates and hybrid arenites are cementation and grain replacement by smectite, zeolites, calcite and dolomite, mechanical compaction and dissolution of feldspars, volcanic fragments and bioclasts. Stevensitic arenites experienced early cementation and replacement of ooids and peloids by quartz, calcite and dolomite, or intense compaction of stevensitic grains in uncemented areas. Volcaniclastic sandstones and conglomerates with smectite rims, remnant intergranular porosity and grain dissolution may constitute fair hydrocarbon reservoirs. Stevensitic and hybrid arenites with dissolution of stevensite grains, bioclasts and calcite cement may also constitute reservoirs, although with potential quality limited by the poor connection of their pore systems. An understanding of the controls on the depositional and diagenetic evolution of the dominantly intrabasinal, gravitationally redeposited, rift succession will contribute to new exploration strategies for the Campos Basin.