Stylolites and depositional cycles in the Thamama B reservoir of the Sahil oilfield, onshore Abu Dhabi

Abstract

Stylolites have been the focus of extensive scientific researches throughout many decades because they can have important effects on reservoir quality and fluid flow. The Thamama B zone is of great economic significance, and stylolites are abundant and widespread in this unit. This petrographic and geochemical study aims to reveal the factors controlling the initiation and development of stylolites in the Thamama B zone by comparing cores from the flanks (water zone) with cores from the crest (oil zone) of a major oilfield. The black residues concentrated along stylolites are characterized using specialized imaging and analysis methods, and the relationship between the sub-zone tops and parasequence-set tops previously defined within the Thamama B third-order-composite sequence is examined. The results suggest that the dissolution surface morphology is largely controlled by the clay content concentrated along the surfaces. The wispy seams and solution seams in the top interval (upper 12 ft of the B zone) from both crest and flank wells have high clay content, which contributed to initializing stylolitization at relatively shallower burial depth, before oil emplacement. The jagged and rectangular stylolites in the center and the basal interval (lower 17 ft of the B zone) from the flank wells, which were well developed due to their location in the water zone after oil emplacement, contain less clay content. The high amplitude jagged and rectangular stylolites in the basal interval (within the transition zone) of the crest wells are distinctive in having thick residues consisting mainly of bitumen and dolomite and these high amplitude stylolites could have been well developed due to the availability of movable water and later oil emplacement into this part compared to the upper part of the B zone. The co-occurrence of higher frequency of stylolites within intervals with lower average porosity indicates the relationship between chemical compaction and reservoir quality deterioration. Regarding cyclicity in the studied cores, the evidence of parasequences is essentially lacking in the Thamama B zone. Occurrences of floatstone/rudstone beds with sharp bases and gradual upward transitions into packstone can potentially be interpreted as parasequence bases. However, the apparent lack of lateral correlation of these intervals supports the alternative interpretation as storm events unrelated to systematic variations in relative sea level. The parasequence-sets corresponding with ADCO reservoir sub-zones are poorly expressed by lithofacies variations in the studied cores. The only basis for recognition of these sub-zone tops that is apparent in the Sahil wells is the negative inflections in the porosity profiles, which correspond in most cases with slight increases in bulk-rock clay content and local occurrences of stylolites (in the flank cores) and more abundant wispy seams (in the crest cores). The high degree of lateral continuity of the sub-zone-top porosity inflections also indicates that they are fundamentally stratigraphic in character. These subtle, but platform-wide, pulses of siliciclastic influx are proposed to reflect times of lower relative sea-level when stream gradients increased. The sub-zone tops are interpreted as resulting from platform-wide depositional variations driven by relative sea-level changes, potentially having significance as fourth-order sequence stratigraphic markers, as proposed by Strohmenger et al. (2006).

Date of Award	2016
Original language	American English
Supervisor	Stephen Ehrenberg (Supervisor)