Paleo Structure Reconstruction to Reveal the Phenomenon of Paleo Hydrocarbon Contact- A Case Study on a Three Major Fields Onshore Abu Dhabi

Abstract

Hydrocarbon presence has been observed in flank wells much deeper than crestal wells in several onshore fields in Abu Dhabi. To explain this phenomenon the integration of paleo-structural reconstruction, petroleum system analysis, and formation evaluation was used. Thickness maps were created between the Early Cretaceous Kharaib-2 (KH-2) and several younger formations that represent paleostructure during the key tectonic and hydrocarbon expulsion events. Formation evaluation that integrates various types of logs, mud logs, geochemistry, core description and analysis have been conducted on several recent appraisal flank wells to confirm the presence of hydrocarbon below the Free Water Level (FWL).The formation evaluation, petroleum system analysis, paleo-structural reconstruction were integrated in order to understand the interesting phenomenon of paleo oil presence below the current FWL. Structural reconstructionshows that various fields have structural crestslocated in the north-east during the Lower Tertiary and remained unchanged during peak hydrocarbon migration inthe Tertiary time.South-west tilting occurred as a result of the Zagros orogeny (Miocene), such that the Lower Tertiary structural crests now lies on the flanks of the fields. As a result of this tilting,hydrocarbon remigration and redistribution occurred and in places oil was replaced by water, but traces of residual oil are still present below the current FWL.This study shows that formation fluid sampling, core and log analysis indicate that the flank wells do not have better reservoir qualities that justify a deeper contact. The study also shows that there is no observed compartmentalization that separates the flank wells with crestal area. Structural reconstruction has explained the poor reservoir properties at the field crest, where it shows that today’s crest wells were flank wells at sometime during structural evolution, and diagenesis was the major playing factor impacting rock quality.