Abstract
Concretions are common features in organic-rich mudstone successions, where they appear aligned along certain stratigraphic surfaces. Their extreme hardness and often large dimensions make them an unwelcome drilling and fracturing hazard in unconventional reservoirs. This study quantifies the number of calcite concretions along stratigraphic horizons in outcrops of the Vaca Muerta Formation of the Neuque = n Basin, Argentina, with the aim of predicting the occurrence in the same horizons in the subsurface. We measured thickness, length, and spacing of more than 800 concretions from 46 different stratigraphic levels of Tithonian and early Berriasian age. Data were gathered along scanlines of variable length in the Sierra de la Vaca Muerta, and Puerta Curaco and Pampa Tril areas. Concretions display significant lateral and vertical variability. Size, spacing, and density index tend to increase toward cycle tops, concomitant with the occurrence of more elongated concretions. In time-equivalent levels at different outcrop locations, larger and more elongated concretions are observed in more proximal settings. In addition to sedimentation rates, an important controlling factor for size and shape of concretions is the availability of calcium carbonate that varies within the sequence-stratigraphic framework and between different areas of the basin. This comprehensive quantitative analysis of concretion occurrence within the Vaca Muerta Formation can help in identifying target intervals less affected by concretion occurrence, and in building more reliable risk-mitigation strategies for drilling and fracturing.
