Fracture Density as a Function of Crystal Size: Insights from a Carbonate Reservoir Analogue

Dati, Francesco ¹; Guerriero, Vincenzo ¹; Iannace, Alessandro ¹; Mazzoli, Stefano ¹; Vitale, Stefano ¹; Giorgioni, Maurizio ²
(1)Earth Sciences, Università Federico II, Naples, Italy. (2) Shell Italia E&P, Rome, Italy.

The relationship between fracture density and crystal size has been analyzed at the microscale in Cretaceous well-bedded carbonates at several sites across the southern Apennines. All the outcrops are characterized by decimeter to meter thick, alternating dolomitic and calcareous beds with variable textures and crystal sizes. Two types of dolomites have been distinguished in terms of petrography and petrophysics, both being related to very early diagenetic processes. The analyzed successions can be considered good analogues of the fractured carbonate reservoirs buried in Val D'Agri and hosting the major oil fields of the Basilicata region (southern Italy). The structural setting of the outcropping analogue successions is characterized by the occurrence of normal faults and two perpendicular sets of associated fractures that developed as a result of extensional tectonics during the foredeep stage, and by reverse and strike-slip faults that formed during the subsequent collisional and post-collisional Miocene to Plio-Pleistocene stages. At the outcrop scale, fracture density is heavily depending on both bed thickness and distance from main faults. The main aim of the study has been to extend the investigation at the microscale for a relatively large number of samples in order to find a relationship between crystal size, lithology and fracture density. Several samples of limestones and dolomites have been selected, showing crystal size ranging between 5 to 50 μm for limestones (wackestones to grainstones), and from 5 to 150 μm for dolomites (fine to medium and coarse dolomite). Microscan lines have been performed in thin sections and acetate peels of selected surfaces, in order to collect statistical data on fracture aperture and spacing. Regardless of the lithology, fine grained samples show very high values of fracture density (from 100 to 250 fractures per meter), whereas coarser samples are characterized by lower values (from 10 to 80 fractures per meter). A power law best fits the data, pointing out an inverse relationship between crystal size and fracture density at the microscale. The results of a detailed analysis on early diagenetic dolomitic beds (characterized by variable texture) alternating with limestones suggests that the role of crystal size largely overcomes that of lithology, very fine grained dolomites behaving similarly to limestone beds of comparable grain size.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.