João Marcelo Ketzer

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.

Dickite in shallow oil reservoirs from Reconcavo Basin, Brazil: diagenetic implications for basin evolution

Abstract Fluvial and aeolian sandstones of the Sergi Formation are the most important reservoirs of the Recôncavo Basin, Brazil. Optical and scanning electron microscopy, X-ray diffraction and infrared spectroscopy revealed the occurrence of dickite, a clay mineral indicative of deep burial conditions (T >100ºC), in the shallow Buracica (630−870 m) and Água Grande (1300−1530 m) oilfields. Vermicular dickite replaces K-feldspar and plagioclase grains, and fills intra- and inter-granular pores. Its vermicular habit is a product of pseudomorphic kaolinite transformation during burial. The presence of dickite is in accordance with the intensity of compaction, post-compactional quartz cementation and δ18 O values of calcite cements (T up to 109ºC). These petrological features of deep burial, as well as apatite fission-track analyses, indicate that uplift and erosion of at least 1 km, and probably >1500 m, affected the central part of the Recôncavo Basin and possibly the entire region. This uplift has not been detected previously by conventional structural and stratigraphic models.

Fluid Seepage in Relation to Seabed Deformation on the Central Nile Deep-Sea Fan, Part 2: Evidence from Multibeam and Sidescan Imagery

On the central Nile deep-sea fan, stratified sediments overlying mass-transport deposits (MTDs) are deformed into slope-parallel seabed undulations associated with fluid seepage. The western part of this system, in water depths of 1,950–2,250 m, is examined using multi-frequency data from hull-mounted and deep-towed swath/profiling systems. Sub-bottom profiles show sub-vertical fluid pipes that terminate both at and below seabed, and gas signatures along fault planes bounding the undulations. Fluid seepage is recorded by high- to intermediate-backscatter patches (HBPs, IBPs) that differ in appearance on multibeam imagery (30 kHz, ≤3 m penetration) and sidescan swaths (170/190 kHz, <0.1 m penetration). Comparison of the two suggests a distinction of (a) buried carbonates (0.1–3 m), (b) broad near-seabed (<0.1 m) carbonate pavements elongate along the undulations, (c) sub-circular areas of seabed seepage up to 300 m across. Four of the latter have narrower gas flares at their edges rising 400–800 m above seabed. These results are consistent with an evolving system of narrow fluid conduits that support the growth and burial of carbonate pavements, shifting over millennial timescales along linear zones parallel to fault planes rooted in MTDs. Sediment deformation above MTDs is inferred to provide pathways for fluid escape, but migration of gas-rich fluids from depth is likely to have facilitated slope destabilisation.

Potencial uso de serpentinito no armazenamento mineral do CO2

Rising anthropogenic CO2 emissions are considered a major contributor to the greenhouse effect. There are several options for reducing atmospheric CO2 levels, and among these alternatives, Carbon Capture and Storage (CCS) has been identified as an effective and promising approach. This work investigated the feasibility of using serpentinite as a vehicle for carbon storage presenting a source-sink match. The main results of the work confirmed that serpentinite is appropriate for the carbonation process due to the high concentration of Mg in its composition.

Post-failure Processes on the Continental Slope of the Central Nile Deep-Sea Fan: Interactions Between Fluid Seepage, Sediment Deformation and Sediment-Wave Construction

Voluminous mass-transport deposits (MTD) have been identified on seismic profiles across the central Nile Deep-Sea Fan (NDSF). The youngest MTDs are buried under 30–100 m of well-stratified slope deposits that, in water depths of 1,800–2,600 m, are characterized by undulating reflectors correlated with slope-parallel seabed ridges and troughs. Seabed imagery shows that, in the western part of the central NDSF, short, arcuate undulations are associated with fluid venting (carbonate pavements, gas flares), while to the east, long, linear undulations have erosional furrows on their downslope flanks and fluid seeps are less common. Sub-bottom profiles suggest that the western undulations correspond to rotated fault-blocks above the buried MTDs, while those in the east are sediment waves generated by gravity flows. We suggest that fluids coming from dewatering of MTDs and/or from deeper layers generate overpressures along the boundary between MTDs and overlying fine-grained sediment, resulting in a slow downslope movement of the sediment cover and formation of tilted blocks separated by faults. Fluids can migrate to the seafloor, leading to the construction of carbonate pavements. Where the sediment cover stabilizes, sediment deposition by gravity flows may continue building sediment waves. These results suggest that complex processes may follow the emplacement of large MTDs, significantly impacting continental-slope evolution.

Environmental monitoring of water resources around a municipal landfill of the Rio Grande do Sul state, Brazil

In Brazil, landfills are commonly used as a method for the final disposal of waste that is compliant with the legislation. This technique, however, presents a risk to surface water and groundwater resources, owing to the leakage of metals, anions, and organic compounds. The geochemical monitoring of water resources is therefore extremely important, since the leachate can compromise the quality and use of surface water and groundwater close to landfills. In this paper, the results of analyses of metals, anions, ammonia, and physicochemical parameters were used to identify possible contamination of surface water and groundwater in a landfill area. A statistical multivariate approach was used. The values found for alkali metals, nitrate, and chloride indicate contamination in the regional groundwater and, moreover, surface waters also show variation when compared to the other background points, mainly for ammonia. Thus, the results of this study evidence the landfill leachate influence on the quality of groundwater and surface water in the study area.

Environmental monitoring of a landfill area through the application of carbon stable isotopes, chemical parameters and multivariate analysis

Leachate produced during an organic matter decomposition process has a complex composition and can cause contamination of surface and groundwaters adjacent to a landfill area. The monitoring of these areas is extremely important for the characterization of the leachate produced and to avoid or mitigate environmental damages. Thus, the present study has the objective of monitoring the area of a Brazilian landfill using conventional parameters (dissolved metals and anions in water) and alternative, stable carbon isotopes parameters (δ13C of dissolved organic and inorganic carbons in water) in addition to multivariate analysis techniques. The use of conventional and alternative parameters together with multivariate analysis showed that cells of the residues are at different phases of stabilization of the organic matter and probably already at C3 of the methanogenic phase of decomposition. In addition, the data showed that organic matter stabilization ponds present in the landfill are efficient and improve the quality of the leachate. Enrichment of the heavy 13C isotope in both surface and groundwater suggested contamination in two sampling sites.

Microbiota associated with tubes of Escarpia sp. from cold seeps in the southwestern Atlantic Ocean constitutes a community distinct from that of surrounding marine sediment and water

As the depth increases and the light fades in oceanic cold seeps, a variety of chemosynthetic-based benthic communities arise. Previous assessments reported polychaete annelids belonging to the family Siboglinidae as part of the fauna at cold seeps, with the ‘Vestimentifera’ clade containing specialists that depend on microbial chemosynthetic endosymbionts for nutrition. Little information exists concerning the microbiota of the external portion of the vestimentiferan trunk wall. We employed 16S rDNA-based metabarcoding to describe the external microbiota of the chitin tubes from the vestimentiferan Escarpia collected from a chemosynthetic community in a cold seep area at the southwestern Atlantic Ocean. The most abundant operational taxonomic unit (OTU) belonged to the family Pirellulaceae (phylum Planctomycetes), and the second most abundant OTU belonged to the order Methylococcales (phylum Proteobacteria), composing an average of 21.1 and 15.4% of the total reads on tubes, respectively. These frequencies contrasted with those from the surrounding environment (sediment and water), where they represent no more than 0.1% of the total reads each. Moreover, some taxa with lower abundances were detected only in Escarpia tube walls. These data constitute on the first report of an epibiont microbial community found in close association with external surface of a cold-seep metazoan, Escarpia sp., from a chemosynthetic community in the southwestern Atlantic Ocean.

Fluid Seepage in Relation to Seabed Deformation on the Central Nile Deep-Sea Fan, Part 1: Evidence from Sidescan Sonar Data

The central Nile Deep-Sea Fan contains a broad area of seabed destabilisation in association with fluid seepage: slope-parallel sediment undulations are associated with multibeam high-backscatter patches (HBPs) related to authigenic carbonates. During the 2011 APINIL campaign, a deep-towed sidescan and profiling system (SAR) was used to acquire high-resolution data along three transects across water depths of 1,700–2,650 m. Three seabed domains are distinguished, all developed within stratified sediments overlying mass-transport deposits (MTDs). Upslope of the undulations (<1,950 m), sidescan HBPs record focused fluid seepage via seabed cracks. In the western area of undulations, sidescan HBPs are distinct from intermediate-backscatter patches (IBPs) that extend up to 850 m parallel to the undulations, mainly along their downslope flanks; some contain sub-circular HBPs up to 300 m wide, three associated with smaller (<10 m) hydroacoustic gas flares. Focused fluid seeps are inferred to have shifted over time to form elongate carbonate pavements, preferentially along the footwalls of faults beneath the undulations that provide pathways for fluid flow. In contrast, in the eastern area of undulations, sidescan imagery reveal only slope-transverse furrows formed by turbulent flows, interpreted to indicate that fossil carbonates sampled during submersible operations have been exhumed by erosion.