Estudo das pressões de poros numa área central da Bacia de Santos
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Data
2017-04-10
Autores
Picolini, João Paulo [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
Foram estudadas as pressões de poros em poços localizados numa área central da Bacia de Santos, onde ocorrem Zonas de Pressão de Poros Anormal (ZPA), intervalos estratigráficos caracterizados por pressões de poros anormalmente elevadas (sobrepressão). Atenção especial foi dedicada à Sequência Deposicional K90, Santoniano ao Campaniano, constituída pelas formações Juréia e Itajaí-Açu, por sua vez composta por pelitos do Folhelho-Selo e arenitos do Reservatório Principal (Membro Ilha-Bela). A área de estudos possui cerca de 1900 km2 e localiza-se a cerca de 120km do litoral do Estado de São Paulo, apresentando acumulações de gás no Reservatório Principal, seladas pelo Folhelho-Selo, entre as quais destaca a Acumulação Principal, com cerca de 800 m de espessura de zona de gás. Os estudos envolveram modelagem petrofísica 1D das pressões e modelagem histórica 1D de parâmetros tectonossedimentares. Objetivou-se, assim, quantificar as pressões de poros; verificar se elas estão relacionadas à evolução tecnossedimentar e sugerir as causas prováveis da sobrepressão. A modelagem petrofísica forneceu curvas de pressões de sobrecarga (PS), pressão hidrostática (PH) e pressão de poros (PP) a partir de perfis elétricos de poços. A modelagem histórica forneceu curvas de subsidência, taxas de sedimentação, fluxo térmico, temperatura e pressões de poros ao longo do tempo. Análises complementares foram realizadas a partir de dados geoquímicos e de mineralogia do Folhelho-Selo. Foi possível caracterizar as pressões anormais nos perfis e, com razoável confiabilidade, estabelecer a distribuição estratigráfica e espacial das ZPAs existentes na área. Na Sequência K90 foi constatada uma ZPA Principal e uma ZPA Secundária, enquanto nas sequências mais antigas (K70, K82-86 e K88) identificaram-se zonas de menor importância e ocorrência localizada (ZPA-2 e ZPA-3). A ZPA Principal engloba a Acumulação Principal e apresenta gradiente de pressão de poros (GPP) de até 12,6 ppg na zona de gás e 11,7 ppg na zona de água, superiores ao gradiente de pressão hidrostática (GPH), cujos valores variam entre 8,5 ppg e 9,5 ppg. A deposição da K90 ocorreu durante importante período de atividade tectônica e taxas de sedimentação de até 300 m/Ma, sendo submetida a fluxos térmicos de até 100 mW/m2 e temperaturas superiores a 140 oC ao longo do tempo. Não foi identificada uma relação entre a sobrepressão da ZPA Principal e a evolução tectonossedimentar da área ou com as características geoquímicas e minerais do Folhelho-Selo. A ocorrência de gás acumulado é o principal elemento que resultou na ZPA Principal, pois o gás elevou GPP da zona de gás em relação ao GPP do aquífero. Além disso, a coexistência de gás e água deve ter reduzido a condutividade hidráulica do Folhelho-Selo e retardado a equalização das pressões com o meio hidrostático, contribuindo com a elevação do GPP no aquífero. Camadas menos permeáveis internas ao Folhelho-Selo e ao Reservatório Principal, além de falhas selantes, reduziram ainda mais o fluxo de fluidos, contribuindo com a elevação das pressões de poros. A migração de fluidos pressurizados, a partir de níveis inferiores, também pode ter contribuído com o aumento das pressões de poros na ZPA Principal.
Pore pressures have been studied through wells located in a central area of the Santos Basin, Brazil, where Overpressured Zones (ZPA) are characterized by abnormally high PP. Special attention was given to the K90 Depositional Sequence, Santonian to Campanian age, constituted by the Juréia and Itajaí-Açu formations, which is composed by pelitic rocks (Shale-Seal) and sandstones (Main Reservoir). The study area is about 1900km2 large, located about 120km far from the São Paulo State coast, and has gas accumulations in the Main Reservoir sealed by Shale-Seal, among which the Main Accumulation stands out, with about 800 m of gas zone thickness. The studies were carried out by 1D petrophysical pressure modeling and 1D historical modeling, aiming to quantify pore pressures and verify whether they are related to tectonossedimentary evolution and to suggest probable causes for overpressure in the area. The petrophysical pressure modeling provided overburden pressure (PS), hydrostatic pressure (PH) and pore pressure (PP) curves from electric well profiles. Historical modeling provided total and tectonic subsidence, sedimentation rates, heat flow curves, temperature and pore pressures over the geological time. Further analyses were carried out from geochemical and mineralogy data from the Shale-Seal. It was possible to characterize the abnormal pressures in log profiles and, with reasonable reliability, to establish the stratigraphic and spatial distribution of the ZPAs in the area. A Main ZPA and a Secondary ZPA were detected in the K90 Sequence, whereas zones of minor importance and localized occurrence were identified, named ZPA-2 and ZPA-3, were detected in the older sequences (K70, K82-86 and K88). The Main ZPA encompass the Main Accumulation and presents a pore pressure gradient (GPP) up to 12,6 ppg in the gas zone and up to 11,7 ppg in the water zone, too much larger than hydrostatic pressure gradient (GPH), whose values range from 8,5 ppg and 9,5 ppg. The K90 was deposited during a period of huge tectonic activity, with sedimentation rates up to 300 m/My and has been submitted up to 100 mW/m2 of heat flow and temperatures above 140 oC through the time. It was not realized a relation between the occurrence of overpressure and the tectonossedimentary evolution of the area or with the geochemical and mineral characteristics of the Shale-Seal. The occurrence of accumulated gas is the main element that resulted in the Main ZPA, because the gas raised GPP of the gas zone in relation to the GPP of the aquifer, which raised the GPP of the gas zone in relation to the aquifer’s GPP. Besides that, the coexistence of gas and water must have reduced the hydraulic conductivity of the Shale-Seal and retarded the equalization of the pressures with the hydrostatic medium, contributing to raising the aquifer’s GPP. Less permeable layers, internal to Shale-Seal, and sealing faults further reduced the flow of fluids, contributing to the elevation of pore pressures. The migration of pressurized fluids, from lower levels, may also have contributed to increased pore pressures in the Main ZPA.
Pore pressures have been studied through wells located in a central area of the Santos Basin, Brazil, where Overpressured Zones (ZPA) are characterized by abnormally high PP. Special attention was given to the K90 Depositional Sequence, Santonian to Campanian age, constituted by the Juréia and Itajaí-Açu formations, which is composed by pelitic rocks (Shale-Seal) and sandstones (Main Reservoir). The study area is about 1900km2 large, located about 120km far from the São Paulo State coast, and has gas accumulations in the Main Reservoir sealed by Shale-Seal, among which the Main Accumulation stands out, with about 800 m of gas zone thickness. The studies were carried out by 1D petrophysical pressure modeling and 1D historical modeling, aiming to quantify pore pressures and verify whether they are related to tectonossedimentary evolution and to suggest probable causes for overpressure in the area. The petrophysical pressure modeling provided overburden pressure (PS), hydrostatic pressure (PH) and pore pressure (PP) curves from electric well profiles. Historical modeling provided total and tectonic subsidence, sedimentation rates, heat flow curves, temperature and pore pressures over the geological time. Further analyses were carried out from geochemical and mineralogy data from the Shale-Seal. It was possible to characterize the abnormal pressures in log profiles and, with reasonable reliability, to establish the stratigraphic and spatial distribution of the ZPAs in the area. A Main ZPA and a Secondary ZPA were detected in the K90 Sequence, whereas zones of minor importance and localized occurrence were identified, named ZPA-2 and ZPA-3, were detected in the older sequences (K70, K82-86 and K88). The Main ZPA encompass the Main Accumulation and presents a pore pressure gradient (GPP) up to 12,6 ppg in the gas zone and up to 11,7 ppg in the water zone, too much larger than hydrostatic pressure gradient (GPH), whose values range from 8,5 ppg and 9,5 ppg. The K90 was deposited during a period of huge tectonic activity, with sedimentation rates up to 300 m/My and has been submitted up to 100 mW/m2 of heat flow and temperatures above 140 oC through the time. It was not realized a relation between the occurrence of overpressure and the tectonossedimentary evolution of the area or with the geochemical and mineral characteristics of the Shale-Seal. The occurrence of accumulated gas is the main element that resulted in the Main ZPA, because the gas raised GPP of the gas zone in relation to the GPP of the aquifer, which raised the GPP of the gas zone in relation to the aquifer’s GPP. Besides that, the coexistence of gas and water must have reduced the hydraulic conductivity of the Shale-Seal and retarded the equalization of the pressures with the hydrostatic medium, contributing to raising the aquifer’s GPP. Less permeable layers, internal to Shale-Seal, and sealing faults further reduced the flow of fluids, contributing to the elevation of pore pressures. The migration of pressurized fluids, from lower levels, may also have contributed to increased pore pressures in the Main ZPA.
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Palavras-chave
Pressão de poros, Sobrepressão, Tectonossedimentação, Bacia de Santos, Petróleo, Pore pression, Overpression, Tectonossedimentation, Santos Basin, Petroleum