Santonian-Campanian Channelized Systems of the Santos Basin, Brazil: Stratigraphic Framework and Reservoir Potential
Published:December 01, 2012
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Diogo Michelon, Edmundo Marques, Jorge Figueiredo, Heitor Ferraz, Paulo Barros, 2012. "Santonian-Campanian Channelized Systems of the Santos Basin, Brazil: Stratigraphic Framework and Reservoir Potential", New Understanding of the Petroleum Systems of Continental Margins of the World, Norman C. Rosen, Paul Weimer, Sylvia Maria Coutes dos Anjos, Sverre Henrickson, Edmundo Marques, Mike Mayall, Richard Fillon, Tony D’Agostino, Art Saller, Kurt Campion, Tim Huang, Rick Sarg, Fred Schroeder
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Three-D seismic and well data enabled the high resolution interpretation of a Santonian-age succession in present day shallow waters of Santos basin, Brazilian east continental margin, where recent oil and gas discoveries have been made by OGX Oil & Gas. An organized hierarchical system of depositional sequences was identified. A lower order composite depositional sequence was interpreted from seismic facies, lithofacies distribution and bio-chronostratigraphy. The composite depositional sequence was broken down into its constituent systems tracts, each of them exhibiting characteristic patterns in seismic and well log data. Comprising the lowstand systems tract of the composite depositional sequence are five stratigraphic levels of sandy deposits intercalated with thick hemipelagic shale drapes which constitute the elementary depositional sequences of this systems tract. The sandy deposits are mainly erosion-confined channels and/or channel complexes that increase upward in both size and net sand content, reflecting the increasing energy associated with the erosive processes that carved the channels/channel complexes. A capping sequence containing smaller channel complexes with less net sand content reflects the waning depositional energy at the end of the lowstand system. Crucial to understand the distribution and connectivity of reservoirs, establishment of this sequence stratigraphic framework enabled the recognition and correlation of individual sandy deposits in each of the elementary depositional sequences, both spatially and chronostratigraphically.
Santos basin is located in the southeastern Brazilian east continental margin offshore of Rio de Janeiro, São Paulo, Paraná and Santa Catarina states. It comprises an area of about 350,000 km2 and is limited to the north with Campos basin and to the south with Pelotas basin. (Fig. 1).
Asmus and Porto (1980) proposed that the Santos basin evolved through three different phases: Lake phase; Gulf phase; and Marine phase. Chang et al. (1992) proposed five mega-sequences reflecting the rift-to-drift evolution of the basin: continental syn-rift, evaporitic transitional, shallow carbonate platform, open sea marine transgression, and regressive cycle. Cainelli and Mohriak (1999), on the other hand, proposed four mega-sequences: pre-rift, rift, transitional and marine. Pereira and Feijó (1994) built up a chrono-lithostratigraphic chart in which they consider that the basin evolved through three tectono-sedimentary super sequences: rift, post-rift, and drift. Moreira et al. (2007) updated this chart keeping the Pereira and Feijó’s concept of super sequence but stepping forward and proposing that these mega-sequences can be subdivided into 25 sequences: three in the rift super sequence, three in the post-rift super sequence, and nineteen in the drift super sequence (Fig. 2).
Exploration in the Santos Basin began in the 1970s. Before the recent discovery of pre-salt giant oil fields in ultra-deep water, the commercial results for hydrocarbons in the basin were almost nil as only minor reserves of gas had been discovered (e.g., the Merluza and Mexilhão gas fields). Contrary to the tendency for exploration companies to build only on recent success in the pre-salt by pursuing exploration strategies targeting ultra-deep-water prospects, OGX Oil and Gas focused on the shallow-water shelf. OGX acquired exploratory blocks about 100 km from the cost in water depths of around 150 m (Fig. 1) which yielded significant discoveries in several different plays. The Santonian Channelized Systems play is one of these. In this paper we propose sedimentary and stratigraphic evolution models for the depositional succession that encompass the channels, channel complexes, and channel complex sets (following Sprague et al., 2003). We establish a new stratigraphic framework and suggest how it can help to mitigate exploratory risk for this play.
The dataset used in this work comprises:
PSTM (Post-Stack Time Migration) 3D seismic data
Regional 2D seismic lines
Data (composite logs, well logs, final well reports) of six exploratory wells drilled in the past acquired from the Brazilian National Petroleum Agency (ANP) • Data (LWD logs, cable logs, CMR, pressure tests, formation tests, thin sections from cuttings samples, rotary side well cores and biostratigraphy) from ten wells drilled by OGX Oil and Gas
Seismic inversion (Ip, Is, Ip-Is) used to interpret the main horizons related to the reservoirs
The scientific methodology applied in this work reflected the exploration methods used at OGX Oil and Gas. Initially, key surfaces were mapped on the whole PSTM (Post-Stack Time Migration) 3D seismic data set. The high resolution mapped surfaces then were matched to selected regional 2D seismic lines into which key older wells acquired from ANP had already been tied. Bio-chronostratigraphic data available in some these wells supported establishment of a preliminary stratigraphic framework for the drift succession in the area of the 3D seismic dataset. This stratigraphic framework provided guidance for the first OGX exploration wells drilled in the basin. The exploratory and commercial success of some of these wells in siliclastic reservoirs of Santonian age indicated that a more detailed analyses of the sandy intervals drilled by the wells would be necessary to evaluate potential reservoir connectivity and seal for the gas/oil field delineation phase of drilling s.
To carry out this new siliciclastic reservoir focused phase of the study at the scale necessary for reservoir continuity studies and to search for possible DHIs, seismic reprocessing was required. A new poststack time migration, pre-stack time migration and pre-stack depth migration were performed. Several seismic attributes and special processing also were carried out: band-with extension (high frequency by Geotrace), inversion volumes (Ip, Is, Ip-Is), MuRho (shear module * density), AVO analysis, coherence volume, spectral decomposition, and others. Of these, inversion volume (Ip, Is, Ip-Is) and MuRho were the most useful for the establishment of a reliable, detailed stratigraphic framework.
The inversion volume was executed only in a selected area and stratigraphic interval comprising the siliciclastic reservoirs of the seismic data set. MuRho was executed, when needed, only in seismic section, not in volume. These two special processing techniques provided high resolution imaging (sections and maps) of the study area sedimentary succession enabling a more confident interpretation on the geometry, architecture and, in some cases, even the internal anatomy of the reservoirs. The architecture of the depositional elements observed in the seismic imaging also enabled us to visualize of depositional patterns and to interpret depositional processes.
Concurrent with this work, high-resolution bio-chronostratigraphic analyses were carried out for the new wells drilled by OGX in the study area. As the chronostratigraphy of each well was determined, it was tied to high-resolution seismic Ip-Is and MuRho sections. These well ties enabled the interpretation that led to the proposed model for the stratigraphic evolution of the study sedimentary succession.
Description and Interpretation at Depositional Sequence Scale
The depositional succession analyzed in this work is shown in well #1 (Fig. 3a). Figure 3b shows a downdip arbitrary seismic section through well #1. Bio-chronostratigraphic data, from nannofossil and forams analyses, in well #1 indicates a major hiatal surface at the passage from the Cenomanian to lower Santonian. Below this surface, the sedimentary section is characterized by intercalated marls and shales, the proportion of shale increasing upward. When matched to the seismic data, this section is seen to match an interval characterized by a predominant “transparent” seismic facies intercalated with some roughly parallel seismic reflectors. Above the hiatal surface, the section is characterized by bright, gently dipping seismic reflectors. In well #1 section above the hiatal surface consists of interbedded shale and sandstone with the sandstone intervals thickening upward, culminating in an approximately 70-meter sandstone body. Above this thick sandstone shale again dominates the section, corresponding to another “transparent” seismic facies interval (Fig. 3b). Above this seismically transparent zone is a series of roughly parallel and short southeast dipping basinward-dipping seismic reflectors. In well #1 this interval ties to a sandy succession.
Fossils in the OGX wells characterize the study section as marine. The major hiatus defined by the bio-chronostratigraphic data is interpreted to span at least, 7.7 m.y., since the Turonian and Coniacian are entirely missing. Below this apparent unconformity, the Albian-Cenomanian succession is characterized by the upward decrease in marls and increase in shale content. This suggests an overall increase in the water depth leading to the interpretation of a marine transgression following Moreira et al., 2007 (Figs. 4a and 4b). When correlated with regional stratigraphic models, not only for the Santos Basin (Moreira et al., 2007), but also for the Campos basin (Winter et al., 2007), the Albian-Cenomanian section shown in Figures 3 and 4 appears to represent the final stage of a long-term transgression. It may be punctuated by higher order events which began in the Brazilian east continental margin drift succession during the early Albian with deposition of shallow-water carbonates and evolved through deep-water carbonates and marls to hemipelagic shales. Above the unconformity, the pattern of the seismic reflectors and the thickening upward sandstone bodies suggest that this section was deposited in an overall regression (Fig. 4b). Because of the contrasting depositional pattern below and above of the unconformity, and the magnitude of the hiatus, we interpret a sequence boundary at this point (Figs. 4a and 4b). The regressive character of the section deposited above the sequence boundary drives the interpretation of a lowstand systems tract (sensu Posamentier and Allen, 2000) in which over time the sedimentary feeder systems moved closer to the well #1 location.
The shale increase above the thickest sandstone body and the change in seismic pattern suggest a change in the depositional systems tract. It is interpreted that this shale section comprises the later stages of a lowstand systems tract and the succeeding transgressive systems tract (Figs. 4a and 4b). The east?-dipping/baasinward-dipping seismic reflectors above this shaly transgressive interval (Figs. 4a and 4b) appear to correspond to the highstand systems tract of the depositional sequence that begins at the unconformity separating Cenomanian and lower Santonian deposits. The upper boundary of this depositional sequence is not clearly defined, either in well data or in seismic data, nevertheless, because the focus of this work is the lowstand systems tract portion of the sequence, lack of a precise sequence boundary location is not an issue.
Lowstand Systems Tract Architecture
In this paper, we focus on the lowstand systems tract of the Campanian-Santonian depositional sequence described in well #1 because it is where the post-salt reservoirs are found. Careful examination of the 3D seismic data and key surface mapping, and well ties reveal that the reflections interpreted as sandstone bodies are irregularly distributed both spatially, within the depositional succession and through time. An along-strike MuRho seismic section tied to well #1 reflecting the elastic properties of the rocks (Goodway et al., 2007) provides the best imagery of the lithological variation in the section. Within the lowstand systems tract it shows five geometric shapes that are matched to sandy intervals drilled by well #1 (Fig. 5). Mapping of these seismically resolvable sandstone body geometries using the high-resolution seismic inversion (IP-IS) volume shows that in plan view these sandstone bodies present a pronounced northwest-southeast oriented linear to curvilinear patterns (Figs. 6a, b, c and d).
Well #1 shows that the five sandy intervals are separated by shaly intervals of about 30-40 m. The exception is the shale thickness between the penultimate (the thickest) and the ultimate (the thinnest) sandy intervals which is more than 50 m. The shaly intervals are usually interbedded with siltstones or very-fine grained sandstone.
The linear to curvilinear plan view geometries of the sandy deposits suggests that they are the confined fill of submarine channels and or channel complexes. Seismic imaging suggests that the channel/channel complexes are constrained mostly by erosion surfaces (Figs. 6a, b, c and d). In some cases, however, it seems that the imaged sandy deposits are unconfined; in these cases they are interpreted as intraslope lobes (following Adeogba et al., 2005 and Fonesu, 2003).
The intercalation of sandy channel/channel complex fills and/or sandy intraslope lobes and shaly intervals in well #1, which is also observed in the seismic data, shows a regular pattern, or cyclicity, suggesting an allocyclic control on deposition. Following Figueiredo et al. (2010), it is interpreted that the lowstand systems tract of the Campanian-Santonian depositional sequence was punctuated by higher order events that conditioned the development of higher order depositional sequences (sensu Schlager, 2004). Each of the five sandy intervals coupled with the overlying shaly intervals are therefore interpreted to represent, respectively, lowstand, transgressive, and possibly highstand deposits of these higher order depositional sequences (Fig. 7a and 7b). As most of the sandy deposits are interpreted as erosion surface confined channel/channel complex fills their thicknesses are controlled by the depth to which flows incised into the erosion surface, which, in its turn, is a function of the energy of the erosive processes involved in the carving of the erosion features. Hence, the thickening upward pattern from the first to the fourth sandy intervals and also the increasing of the amount and size of the sandy depositional elements suggest an increase in the energy of the events responsible for the erosive phase during the building up of the lowstand deposits of the higher order depositional sequences. As the lowstand deposits of the fourth (the penultimate) higher order sequence are the largest and the thickest, it is interpreted that this was the time of the highest energy in all the lower order Campanian-Santonian depositional sequence. Therefore, it can be said that the erosion features of the penultimate higher order sequence happened during the ‘F’ inflection point of the lower order sequence (Fig. 8).
Following Schagler (2004), it is assumed in this work that, as far as allocyclic nature of higher order events can be demonstrated, depositional sequences are arranged fractally. Therefore, the object of this work, the main reservoir for hydrocarbons in the area, must also be expected to behave fractally. Despite being deposited within the lowstand systems tract of a lower order depositional sequence, the reservoirs are, in fact, within the lowstand deposits of the higher order depositional sequences, which in this case study constitute the basic building blocks of the stratigraphic framework. At present, with the currently available data sets it is not possible to break down the geological record any farther. This being the case, the analyzed depositional succession is hierarchical, following Figueiredo (2010) into an elementary depositional sequence (higher order) and a composite depositional sequence (lower order).
The establishment of the stratigraphic framework of the Santonian-Campanian depositional succession in shallow waters of the Santos basin has led to the identification of a composite depositional sequence in which its lowstand systems tract is composed of five elementary depositional sequences. Each elementary depositional sequence lowstand systems tract consists of sandy erosion-confined channel and channel complex fills that are the reservoirs for hydrocarbons. The understanding of the spatial and temporal distribution, and geometry of the sandy reservoirs, as well as the erosional/depositional processes involved in their formation that we developed in the course of this work, is helping to mitigate risk in OGX’s exploration and reserve delineation programs.
The authors acknowledge OGX Oil and Gas for release for publication the data presented in this work. The authors also thank the managing body of the Company for the professional and personal support and the incentive to carry out the research that ended in this work. The original manuscript of this article was improved by constructive reviews by the Executive Director of GCSSEPM Dr. Norman C. Rosen, and two other anonymous reviewers.
Figures & Tables
New Understanding of the Petroleum Systems of Continental Margins of the World
- Atlantic Ocean
- clastic rocks
- continental margin
- geophysical methods
- geophysical profiles
- geophysical surveys
- petroleum exploration
- reservoir rocks
- Santos Basin
- sedimentary rocks
- seismic methods
- seismic profiles
- sequence stratigraphy
- South America
- South Atlantic
- Upper Cretaceous