Análise estratigráfica integrada do Albiano ao Campaniano da Bacia Potiguar: da escala sísmica à de reservatórios

Abstract

The Albian to Campanian interval of the Potiguar Basin is very important academicaly and economicaly. However, there is no complete and updated seismic scale stratigraphic framework for this interval that describes its sequences and system tracts. Additionally, on a reservoir scale, the development of producer oilfields in the Açu Formation is based on lithostratigraphic zonations that do not always represent heterogeneities critical to the fluid flow in the reservoirs, which increases the geological uncertainties associated to the characterization of producer zones and producer well’s behavior. Aiming to solve such demands, the present thesis aims to provide an integrated stratigraphic framework for the studied interval, which describes sequences, stacking patterns and stratigraphic surfaces in the different scales of observation. At the seismic scale, an integrated analysis of seismic, well and biostratigraphic data led to the definition of five 3 rd order depositional sequences and their respective system tracts and stratigraphic surfaces. The Depositional Sequence 1 comprises Albian siliciclastic and carbonate deposits. The Falling Stage System Tract of Depositional Sequence 1 (FSST 1) is preserved in submarine canyon C1. The Lowstand System Tract of Depositional Sequence 1 (LST 1) is composed of fluvial deposits restricted to valleys or non-confined and amalgamated and deltaic deposits basinward. Marine carbonate rocks (Ponta do Mel Formation) and their onshore fluvial counterparts represent the Transgressive (TST 1) and Highstand (HST 1) system tracts. Depositional Sequence 2 are mid- to early Cenomanian aged. The submarine canyon C2 is filled with FSST 2 and LST 2. Onshore, LST 2 is composed of braided fluvial systems. TST 2 onlaps the Maximum Regression Surface (MRS 2) in the marine section and is composed of meandering and tide influenced fluvial deposits onshore. The HST 2 downlaps the Maximum Flood Surface 2 (MFS 2) and is very restricted onshore. Depositional Sequence 3 is late Cenomanian to mid-Turonian aged. TST 3 onlaps MRS 3 in the marine sections and is composed of meandering fluvial and estuarine deposits onshore. HST 3 downlaps MFS 3 in the marine section and is composed of carbonate deposits and their onshore siliciclastic counterparts. The depositional sequences 4 and 5 are entirely inserted within the context of the carbonate platform of the Jandaíra Formation and only transgressive and highstand system tracts were interpreted. Açu Formation, the main oil-bearing formation of the Potiguar Basin, is inserted into depositional sequences 1, 2 and 3 and is strongly affected by the lateral and vertical variations of the depositional systems. This low-resolution stratigraphic framework was used as reference for the stratigraphic analysis in reservoir scale of the onshore portion of TST 2 – in the fluvial domains of Açu Formation – and of the offshore portion of HST 1 – in the carbonate domains of Ponta do Mel Formation. The integrated analysis of rock, well logs and production data from a mature heavy oil field led to the interpretation of nine 4 th order depositional sequences that compose the TST 2. Such depositicional sequences are asymmetrical cycles where only the transgressive facies tracts are preserved, which are composed of a vertical sucession of channel and floodplain fluvial deposits, bouded at top by paleosol levels. The paleosols are the only register of the 4 th order regressive events and are the diagnostic features for the interpretation of the sequence boundaries. The paleosol levels and, consequently, the subaerial unconformities, are correlated throughout the oil filed and control the fluid flow within reservoirs, constituting their main vertical heterogeneities. Thus, this high-resolution chronostratigraphic framework and the LST 2 were used as the new reservoir zonation, replacing the former four lithostratigraphic zones. The new reservoir zonation allowed a more detailed reservoir characterization and led to a better representation of fluvial systems in 3D geological models and to the correction of mapping errors. The understanding of the fluid flow pattern within the reservoirs led to the closing of intervals that are harmful to production, as well as to the identification of remaining oil not produced, responsible for increasing the useful life of wells submitted to cyclic steam stimulation. Such results would not be obtained without the high-frequency chronostratigraphic zonation and reinforce the importance of high-resolution sequence stratigraphy as a tool for the improvement of mature oil fields recovery factors. The same methodology was used for the stratigraphic refinement of the carbonate deposits of HST 1, subdivided then into four 4th order genetic sequences. These sequences are composed of regressive and transgressive facies tracts which represent different paleogeographic configurations of the stacked carbonate platforms that compose the HST 1. Even though the HST 1 is not an oil-bearing interval, the proposed high-resolution chronostratigraphic framework is a good analogy for the study and description of stratigraphic heterogeneities in marine carbonate reservoirs. The multiscale methodology used in this work can be replicated in other stratigraphic intervals of Potiguar Basin, as well in any other sedimentary basin worldwide, where similar results can contribute to oil industry both in exploratory and reservoir scales.