One of the biggest challenges when it comes to predicting reservoir behaviour is the correct interpretation of flow barriers. The most important question that always comes up is: “How laterally extensive are the barriers found in wells?”, as this can determine reservoir performance and sweep efficiency.
In the Lower Eocene paralic sandstones of the Cambo oil field (UK Atlantic margin), wells have found local calcite-cemented intervals varying in thickness from 10 cm to over a metre. Kevin Purvis (Siccar Point) and co-workers used a novel technique that has not been used to address an oilfield query before to further analyse these cemented zones and make inferences on their lateral extent.
The Cambo field, which contains over 800 million barrels of oil, is located in the Faroe-Shetland Basin on the Atlantic margin. It was discovered in 2002 by Hess on a large basement high at the southern edge of the Corona Ridge. Five appraisal wells were drilled prior to acquisition by Siccar Point through the purchase of OMV in 2017. Shell farmed in in 2017 and the partnership drilled one more appraisal well in 2018, which was successful. The field is operated by Siccar Point Energy E&P Ltd (70%) on behalf of joint venture partner Shell UK Ltd (30%) and a final investment decision is expected in 2021.
The Hildasay sands are poorly consolidated and typically have excellent reservoir quality sandstones with average core porosity of >30% and multi-darcy permeabilities. Oil present in the Cambo Field is moderately biodegraded with a gravity of 22-25 API and a viscosity of 3-12 cP at reservoir conditions.
“Many studies into calcite cementation use stable isotopes to determine the chemistry of the pore waters during precipitation,” Kevin explains. “However, this technique does not offer a unique solution as the isotopic signature depends on temperature and water source. The technique we used is called clumped isotope analysis, which offers a unique solution to determine the temperature and pore water composition when the cement formed,” Kevin adds.
The results of the analysis show that the calcite formed at a depth of around 1 km, when temperatures reached ~40-50°C. The consistency in the temperature of formation based upon the clumped isotope analysis suggests that the calcite formed as a single event.
It was also found that the cemented horizons always centre around a thin lag of finer grained material that contains woody material, which most likely formed the nucleus for a chain of diagenetic reactions that ultimately caused the calcite cement to form.
“These fine-grained lag deposits within the estuarine channel sands are thought to have a limited lateral extent given their constant reworking by subsequent phases of estuarine channel activity and the deposition of coarser grained sands,” Kevin and his team concluded.
Therefore, it has been suggested that the cemented zones are discrete layers but with a limited lateral extent. This type of interval may even improve sweep efficiency because the flow paths from the injectors to producers are more tortuous through the reduction of the reservoir’s effective vertical permeability.
Kevin and his co-workers published the results in Marine and Petroleum Geology recently, see the link to the paper below.
Purvis, K., Dennis, P., Holt, L., Marca, A., The origin of carbonate cements in the Hildasay reservoir, Cambo Field, Faroe-Shetland Basin; clumped isotopic analysis and implications for reservoir performance, Marine and Petroleum Geology (2020), doi: https://doi.org/10.1016/ j.marpetgeo.2020.104641.