The nature and significance of sand intrusions in a hydrocarbon-rich fold and thrust belt: Eastern Carpathians Bend Zone, Romania

Abstract

Sand intrusions are commonly associated with hydrocarbon-bearing clastic reservoirs and they can host a significant part of the reserves. They modify the depositional sandbody architecture, and can have an impact on the petroleum system elements. Their significance and complex architecture is difficult to assess in the subsurface due to limited resolution of seismic or core data. Outcrop analogues are key to better understand these complex reservoirs. In the Eastern Carpathians Bend Zone, the Oligocene and lower Miocene succession offer spectacular exposures of deep-water siliciclastic rocks modified by remobilization and sand-intrusion processes. These rocks are also present in producing fields in this area. Sills can be as thick as the depositional sandstones (c. 4 m), and sill-dominated outcrops can provide a net:gross (net sandstone to shale ratio) of about 35–50%. We quantified their architecture, in a study of the timing and mechanism of emplacement, their relationship to regional tectonics and their impact on reservoir connectivity. The intrusions are interpreted to be syn-tectonic, related to the major mid-Miocene compression in this part of the Carpathian Mountain Belt. The intrusions lead to a large increase in vertical and horizontal connectivity. Our results will help to improve reservoir characterization and production in these reservoirs.