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Opening-Mode Fracturing and Cementation Timing in the Barnett Shale, Delaware Basin, West Texas


Natural fractures in shale hydrocarbon reservoirs can impact production by interacting with hydraulic fractures, and potentially contributing to permeability. They are common although their distribution is heterogeneous and they form due to different mechanisms operating before, during and after hydrocarbon generation. Fracture-filling cements provide information that can help constrain the timing and mechanisms.

Analysis of fracture-filling cements in three fracture sets from a Barnett Shale core, Pecos County, Delaware Basin, West Texas is combined with a 1D burial history model, and fluid inclusion microthermometry and Raman spectroscopy, to determine fracture timing and mechanism of fracture formation. An early fracture set, folded during shale compaction, is mostly sealed with dolomite, but retains small pores lined with barite. A second set includes bed-parallel, low angle, and irregular subvertical fractures that are sealed with fibrous barite. The barite contains coexisting primary, liquid-rich hydrocarbon (oil) and aqueous fluid inclusions trapped at ~110°C, and ~55 MPa, respectively. We interpret Set 2 fractures and fill formed during rapid burial when compaction disequilibrium, combined with cracking of type II kerogen to oil, caused an overpressure, thus providing a mechanism for fracturing. A third set of fractures, which are partly open, contains quartz cement bridges with crack-seal structure indicative of cementation during episodic fracture opening. Quartz cementation continued after opening had ceased, overgrowing the crack-seal structure. Methane saturated aqueous inclusions in the quartz bridges formed at P-T conditions of ~110°C and 35-45 MPa in the crack-seal quartz, and ~128°C and ~35 MPa in quartz overgrowths. Correlation with a burial history model suggests this set opened concurrently with, or soon after, the fractures with fibrous barite. Secondary vapor-rich hydrocarbon inclusions are absent in some Set 3 fractures but are present in others; they are also present in the barite that lines pores in Set 1. Subsequently barite and Fe-dolomite cements were deposited in inactive Set 3 fractures. We conclude that all three fracture events likely formed during burial; first in incompletely compacted mudrock, second by disequilibrium compaction accompanied by oil generation and third through episodic growth close to maximum burial although elements of the third set may have developed during uplift.