Seismocracks: Tectonic Origin of Stylolites, Abu Dhabi Carbonates

Abstract

Abstract Stylolites have been defined as irregular planes of discontinuity at the bedding boundaries caused by pressure solution, which leads to concentration of relatively insoluble materials of the surrounding/enclosed rocks. Factors controlling stylolite formation, and the relationship between stylolites and the porosity of the enclosing reservoirs were subject of many studies. The published literatures identified the pressure solution as the carbonate diagenesis that influenced by mechanical compaction, chemical changes, water flow and cementation. The pressure extension within the carbonates is affected by the grains size, shape, and type, in addition to the clay content and temperature. Others adding the role of stress, which is responsible for stylolites development and distribution. This paper justifies, the tectonic origin of stylolites. The stylolites were studied in many carbonate reservoirs of Abu Dhabi fields and comparing the cores with extensive outcrop studies. The collected data are also investigated from the water zone, on the flanks of the structure with cores from the oil zone, on the crest. The main controlling factor of stylolites initiation and propagation is at the tectonic boundaries and tectonic events showing its tectonic origin. The present study addresses a unique approach to solve this feature and hence understanding its impact on hydrocarbon flow, migration and trappment. The development of stylolites creates a reduction in the porosity, permeability and the bed thickness of the reservoir, the latter were not observed in Abu Dhabi as there is no bed thickness reduction were addressed. Stylolites are principally serrated surfaces formed due to tectonic stress events and their amplitude is dependent on the stress dissipation stages. In addition, it is found some stylolites played as microfaults, which justifies the tectonic origin. This contradicts the previous interpretation, which justifies the needs of compaction, although compaction needs weight of overburden, while the pressure solution process happening in shallow depths. As the fluids migrates, two situations arising, the first is when the maximum stress is vertical, where along the serrated shearing surfaces results in dissolving and accelerating the crests left behind leading to dissolution propagation. The second when the maximum stress is parallel to the bedding, leading to slowing down the crests propagation left behind. In case, the bedding planes are not sheared enough, the connectivity is very less leading to trapped fluid, which facilitate dissolving and penetrating the serrated areas along the minimum stress direction. The results should be relevant to unravel and predict: the spatial and temporal distribution of Seismocracks and their roles in reservoir performance and production. The Seismocracks are thus of first-order importance for reservoir performance.