Quantitative Assessment of Regional Siliciclastic Top-Seal Potential: A New Application of Proven Technology in the Pelotas Basin, Offshore Brazil

Abstract

Using the offshore Pelotas Basin, Brazil, as an example, we present a methodology by which petrophysically‐derivedVclay and capillary displacement pressure data, in conjunction with interval isochore maps, can be used quantitatively to evaluate regional, siliciclastic present‐day and palaeo‐top‐seal effectiveness and relative risk. This method has broad application to frontier and maturing exploration areas where data limitations preclude more sophisticated seismically‐derived velocity‐based evaluations. As much of the Pelotas Basin is deemed to be gas prone, top‐seal effectiveness for normal density (0.1 to 0.2 g/cc) dry gas was assessed quantitatively by establishing relationships between density‐log ‐derived hydrocarbon column height and overburden thickness using a most likely Vclay content. With constant Vclay, column height increases with increasing overburden due to a compaction‐driven decrease in mudrock porosity, accompanied by a decrease in permeability and pore‐throat diameter. Using these relationships, interval isochore maps (overburden thickness maps) can be transformed into hydrocarbon column‐height maps to define spatial variation in top‐seal effectiveness, expressed in metres of contained gas column. Laboratory and previously‐published model data show that clay content and porosity (ultimately pore throat diameter) are the dominant controls on siliciclastic mudrock permeability, hence top‐seal potential. Mudrock porosity is driven dominantly by burial‐induced compaction; clay content is dependent on both depositional and diagenetic processes. Overburden can be determined with a reasonable degree of certainty from seismic and well data, whereas regional variations in clay content can, at best, only be estimated from depositional models. Therefore, for a given overburden, it is uncertainty in clay content that comprises the greatest risk in regional siliciclastic top‐seal analysis. For this reason, we relate siliciclastic top‐seal risk to clay content, when overburden/ mudrock‐porosity relationships, fluid densities and requisite column heights are known.

Seismic stratigraphy and subsidence analysis of the southern Brazilian margin (Campos, Santos and Pelotas basins)

“…In addition, intercalated lenticular-shaped bodies of sandstones may represent potential stratigraphic traps. Although most of the oil discoveries in the Pelotas Basin remain uncommercial, bottom simulating reflectors (BSR) may indicate gas hydrates accumulations across the shelfebasin margin transition (Fontana, 1989;Fontana and Massumeci, 1994;Deckelman et al, 2006;Santa Ana et al, 2008). On the Pelotas middleelower continental slope (2000e4000 m of water depth), two high-amplitude reflectors extend almost parallel to the sea-bottom surface and cut through the stratified EoceneeMiocene succession (Figs.…”

“…14) indicate that the oil-window has been reached since the Early Oligocene. Cretaceous mudrocks constitute quality top-seals of potential reservoirs (Deckelman et al, 2006). As indicated before, salt-induced deformation in the Campos Basin seems to reduce the overburden thicknesses and delays post-rift source rock maturation.…”

Biocronoestratigrafia da Bacia de Pelotas: estado atual e aplicação na geologia do petróleo

“…Os possíveis prospectos da Bacia de Pelotas são turbiditos cretácicos e terciários, especialmente aqueles de idade miocênica depositados no Cone do Rio Grande, onde foram verificadas fortes evidências da presença de hidratos de gás, além de ter sido comprovado o potencial para acumulação de gás (Fontana 1989, Sad et al 1998, Deckelman et al 2006, Rosa 2007. Estes depósitos apresentam características como a configuração, detectada através de seções sísmicas por Rosa (2007) e uma porosidade estimada por Deckelman et al (2006) de 25 a 30%, que os caracterizam como favoráveis ao acúmulo de hidrocarbonetos.…”