Sergipe-alagoas basin

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Introduction

The Sergipe-Alagoas Basin is a basin located on the continental margin of the northeast region of Brazil that covers the states of Sergipe, Alagoas and parts of the state Pernambuco. The basin has a total area of around 44,000 km^2, ~32,000 km^2 offshore and ~12,000 km^2 onshore. The Sergipe-Alagoas basin has had many companies invest in the region due to the lucrative reservoirs that contain liquid petroleum and natural gas. The basin has been actively explored for almost 100 years. The basin is split into two sub basins, Sergipe in the south and Alagoas in the North. New areas are still being explored and found in the basin today and the basin has a ton of untapped potential.

This figure outlines the Sergipe-Alagoas Basin located in Northeast Brazil.[1]

History

The Sergipe-Alagoas Basin has had activity for around 100 years dating back to the early 1950’s. In the 50’s the basin was more sporadic with less certainty and low production. The 60’s brought heightened drilling in the area and the largest onshore field named Carmopolis was discovered. In the 1970’s the region became even more active when notable offshore seismic and drilling operations began, and the 110 MMboe Caioba field was found. The 80’s to late 2000’s brought continued exploration with minor discoveries, but in 2010 the offshore section of the basin brought new exciting discoveries.[2] Petrobras, a major oil and gas company in Brazil, has recently found new oil fields in the Sergipe-Alagoas basin. The oil fields have been named Budião, Budião Noroeste, Budião Sudeste, Palombeta, Cavala, Agulhinha, and Agulhinha Oeste. They plan to develop these fields through two modules, Sergipe Deepwater I and II (SEAP), which will overlook the installation of the FPSO-type platforms. The first platform will be the P-81, with a capacity to produce 120 thousand barrels of oil/condensate and flow 8 million m³ of gas per day. This platform is scheduled to start production in 2026 and will serve the SEAP module I. The second platform, planned to serve the SEAP II module, is in the contracting planning phase and is scheduled to start production after the 2022-2026 Strategic Plan horizon.[3]

Geologic Setting

Stratigraphy of the Sergipe-Alagoas Basin.[4]

The Sergipe-Alagoas Basin's geologic setting started with the rifting of South America from Africa but mainly occurred during the drift stage. The drift stage, which began during the Early Albian, established a shallow calcareous platform. Fan-deltas were deposited along faulted borders, in the highs limestones with ooids and oncoids were formed, while in the lows calcilutites and shales were deposited. The shallow platform was then drowned by a trangression, when a mixed carbonatic to siliciclastic ramp was deposited, recorded by calcareous mudstones and shales. Afterward, a regressive event resulted in the Sub-Calumbi Formation Unconformity, later covered by siliciclastic marine basin shales and deltaic to turbiditic deposits (Calumbi Formation), coastal sandstones (Marituba Formation), and bioclastic limestones (Mosqueiro Formation) of the Piaçabuçu Group. The sedimentation trend was transgressive between Santonian and Early Campanian, and subsequently, was mainly regressive.[5]

Petroleum Elements:

Source Rock and Migration

In the basin most, if not all, of the petroleum generating potential can be attributed to the Muribeca Formation. Up to 50m of Bituminous, dark to brown shales, with locally numerous plant and animal remains, have wide areal distribution at the base of the evaporitic Ibura Member. They show the first signs of a basin restriction which led to the extreme local precipitation of sodium, potassium, and even magnesium salts. Therefore, environmental conditions were sufficient for the preservation of organic matter. Another line of evidence consists of the fact that wherever the Ibura Member lies upon the basement, fractures are found to contain oil. Therefore, it seems safe to credit multiple petroleum indications in the Carmopolis Member, as contrasted with an almost complete absence of shows int other lithologies of the studued sequence, to the good source rock potential of the basal shales in the Ibura Member. Proximity between source and reservoir rocks indicated primary migration processes. It allowed the expelled fluids to be displaced directly downward into the reservoirs. Movement of the oil into the structural-stratagraphic traps of Siririzinho, Carmopolis, and 32 Riachuelo also must have involved migration from the adjoining Siriri-Divina Pastora, Jaratuba, and Treme grabens.[6]

Seals, Reservoirs, and Traps

The Sergipe-Alagoas Basin has a large assortment of seals, reservoirs, and traps around the basin. It has traps that involve faults, folds, and unconformities such as intrusive induced anticlines, unconformity truncation play, fault controlled 3 way dip closure, and more. The basins seals include rocks such as Aptian shales, Late Cretaceous shales, Intra formational shales, and more. The reservoirs contain Aptian sands and imsts, Early Cretaceous syn-rift fluvio deltaic and alluvial sands, and much more.

Types of traps, seals and reservoirs in the Sergipe-Alagoas Basin.[7]

Potential

The Sergipe-Alagoas basin has a ton of untapped potential left with new discoveries being found to this day. Recently through the use of seismic data and more modern methods, companies are finding out that the undrilled offshore potential in the basin far surpasses the discovered resources to date. Recently, a Brazilian company named Petrobras has discovered crude oil with an API of 38-44 in the Sergipe-Alagoas region. They believe the oil is very good and very light, and since this basin doesn't require expensive drilling through a sub-salt layer like other areas in the basin, it is less costly to develop. These new and recent discoveries have analysts changing their earlier predictions of the basin. Before they had thought the basin only had shallow water reservoirs that could have 500 million to 1.5 billion barrels, but now they believe the deepwater portion could hold an extra 1 billion barrels or more. [8]