Marco A. S. Moraes

Diagenetic Heterogeneity and Reservoir Quality: Fluvial, Deltaic, and Turbiditic Sandstone Reservoirs, Potiguar and Reconcavo Rift Basins, Brazil

Fluvial and lacustrine (deltaic and turbiditic) sandstones are major hydrocarbon reservoirs in the onshore (Lower Cretaceous) Potiguar and Reconcavo rift basins in northeastern Brazil. Diagenetic elements including mechanically infiltrated (MI) clay, calcite, dolomite, and chlorite show distinct distribution patterns at interwell and field-wide reservoir scales. Such distributions, and thereby diagenetic heterogeneity, have been modeled based on thin-section, core, well-log, and petrophysical data. At the interwell scale, relationships between sandstone body geometry and the distribution of diagenetic elements are a critical aspect of reservoir heterogeneity. At this level, (1) MI clay concentrations occur in fluvial reservoirs following the orientation of paleochannels, generating strong compartmentalization of the reservoirs, (2) calcite cement appears either concentrated near sand-shale contacts (peripheral distribution) or dispersed in the interior of the sandstone layers (scattered distribution), (3) dolomite cement may appear concentrated along laminations in cross-stratified sandstones, causing a decrease of one order of magnitude in the effective horizontal permeability, and (4) authigenic chlorite is observed to reduce more significantly the permeability in fine-grained sandstones than in coarse-grained sandstones, an effect that may cause important modifications in the permeability structure of the reservoirs. At the field-wide scale, diagenetic heterogeneity includes (1) stratigraphic zonation of diagenetic properties in fluvial reservoirs, (2) increasing carbonate cementation toward the border of the hydrocarbon accumulations in deltaic reservoirs, and (3) distinct patterns of carbonate cementation observed in channel fills and lobes in turbiditic reservoirs. Incorporating the distribution and effects of diagenetic elements into the interwell and field-wide geological modeling is essential to achieve a realistic reservoir representation. Such incorporation is performed by the integration of diagenetic properties with other geological attributes, including nature and distribution of depositional facies, structural elements, and stratigraphic framework. This procedure improves reservoir quality evaluation and leads to more precise prediction of reservoir performance.

Characterization of Thin Beds Through Joint Time-frequency Analysis Applied to a Turbidite Reservoir In Campos Basin, Brazil

A new spectral decomposition based method is presented here. We propose to use the ridges of the joint timefrequency analysis, as a new way to detect, in each trace, the maximum instantaneous frequencies and their associated amplitudes, which were applied as a tool to detect seismic geomorphologic bodies. The technique was applied to a synthesized wedge model and also to a thin bed offshore turbidite reservoir in Campos Basin, Brazil.

Diagenetic Evolution of Cretaceous-Tertiary Turbidite Reservoirs, Campos Basin, Brazil

Three sandstone turbidite sequences of the Campos basin (offshore, state of Rio de Janeiro, Brazil) were petrologically studied: (1) the Albian-Cenomanian Namorado Sandstone on the Macae Formation, (2) the Upper Cretaceous Carapebus Member of the Campos Formation, and (3) the Eocene Carapebus Member of the Campos Formation. The sequences represent deep marine deposits consisting mostly of massive sandstones rather than classical turbidites, indicating sand-rich submarine fans were the main depositional system of these sequences. The framework composition of the sandstones averages for quartz, feldspar, and lithics are Q60F40Ltr for the Cretaceous rocks and Q71F29Ltr for the Eocene rocks, plotting granitic r ck fragments at the feldspar pole. The main diagenetic phases that affected the sandstones studied were (1) development of a clay matrix due to compaction of rip-up mud clasts, (2) partial replacement of the matrix by opal, (3) precipitation of small pyrite framboids, (4) widespread direct precipitation or replacement of different materials by calcite, (5) intense generation of secondary porosity, (6) localized kaolinite development, (7) minor precipitation of quartz and feldspar overgrowths, (8) development of dolomite, ferroan dolomite, and ankerite, and (9) replacement of different materials with minor direct precipitation of late pyrite. Geologic and geochemical evidence lets us infer the main processes that controlled diagenetic transformations and mass transfer within the sequence studied. The principal source of carbonate cements was pressure solution of the underlying Macae Formation. Most of the diagenetic evolution of the sandstones was apparently controlled by the relative balance between the activity of CO2 and carboxylic acid species in the formation waters, both related to organic matter transformations within adjacent marine shales. The timing of hydrocarbon (generated in deeper lacustrine shales) migration to the reservoirs was an important factor in preserving diagenetically enhanced porosity.

Diagenesis and Microscopic Heterogeneity of Lacustrine Deltaic and Turbiditic Sandstone Reservoirs (Lower Cretaceous), Potiguar Basin, Brazil (1)

Most of the hydrocarbon reservoirs in the Pendencia Formation, onshore Potiguar basin, northeastern Brazil, are fine- to coarse-grained deltaic and turbiditic lacustrine feldspathic sandstones which contain quartz and feldspar overgrowths, calcite, secondary porosity, dolomite, and chlorite as the principal diagenetic elements. Porosity in these rocks is a complex association of reduced primary porosity (5%), carbonate dissolution porosity (7%) and framework-grain dissolution porosity (6%). Each porosity type displays characteristic textural features which permit recognition and quantification in thin sections. The secondary porosity apparently was produced by corrosive fluids generated from adjacent or deeper shales. The effect of the diagenetic phases on permeability was distinct from their effect on porosity. Initially, quartz and feldspar overgrowths partially obstructed the pore throats, significantly reducing permeability. Later, replacement by calcite of grain or overgrowth boundaries followed by calcite dissolution generated the 8-micrometer-diameter pore throats that control the present permeability system. Among the late diagenetic phases, chlorite was the most harmful to reservoir permeability, affecting especially fine-grained sandstones. The present pore-fluid distributions in the Pendencia reservoirs show that high irreducible water saturations (S[wi] around 40%) are common. These high S[wi] have generally been attributed to chlorite aggregates. However, analysis of the pore structure reveals that microscopic heterogeneity related to the geometrical relationship between preserved primary porosity and diagenetically produced secondary porosity is the main cause of the observed high water saturation. Irreducible water tends to remain held within discontinuous patches of primary porosity as oil fills a continuous network of secondary porosity.

Reservoir Representation for Flow Simulation

Reservoir flow modeling involves two aspects: a functional model (flow equations and numerical methods) and a representation model (related to the reservoir description) – and this last aspect is usually the critical one. Reservoir representation is different from reservoir characterization. It is not a question of describing the reservoir in a more or less exhaustive or realistic way, but a question of incorporating relevant information into the flow simulation model, considering the syntax of the flow simulators and the relative impact of information on the simulation results. Reservoir representation makes the bridge between reservoir characterization and flow simulation. This paper presents selected results from a cluster of applied research projects in reservoir representation for flow simulation. It includes: (1) gridding issues in the context of integrated reservoir studies (discussion of terminology, grid specification, generic format reading, etc.); (2) identification and representation of critical heterogeneities of turbidite reservoirs; (3) assigning transmissibility multipliers across partially sealing faults in a field case; (4) incorporation of production data in reservoir flow models with an example of application; (5) hyperdocumention for reservoir simulation files (making use of HTML tags to get a much richer documentation of the flow simulation model).