IMPLICATIONS OF STRUCTURES IN UNORIENTED CORES OF UTICA BLACK SHALE

We performed detailed structural analyses on 713.5 m (2341 ft) of Ordovician Utica black shale core from 7 boreholes in the Mohawk Valley region of NYS. We documented 1148 veins and 3 clean fractures, including 58 veins and fractures with slip described in the table below. Sense of slip was deduced from releasing bends (rhombochasms) in calcite-filled fractures, bedding offsets, and slickenside/fiber orientation. That normal, thrust, and oblique slips are observed on fractures are consistent with foreland basin and escape tectonics.

core #	Fault Name	core dist. to fault (km)	throw (m)	Dist. to thrust front (km)	Utica recovered in core (m)	Total Veins⁺	Slip
							Normal	Thrust	Oblique/Strike Slip	Bedding Plane Slip	Dip Slip unresolved motion
75NY2	Saratoga-McGregor	0*	137	8	137.4	328	11	16	0	0	0
75NY11	Hoffmans	1.73	381	23	113.7**	46	2	1	1	0	4
74NY12	Hoffmans	0.55	381	23	53.6	412	0	0	1	0	1
74NY11	Fonda	0.87	>40	45	3.7	142	1	1	1	1	1
74NY9	Fonda	2.54	>40	45	101.5	134	1	0	6	1	0
74NY10	E. Stone Arabia	1.46	24	58	83.2	74	4	4	0	0	0
74NY5	Mother Creek	3.97	137	68	220.4	12	0	0	0	0	0

*Core is on a fault splay (Tice, 1993)

**Represents core examined. Total recovered Utica is 163.19 m

⁺No veins are dated

Factors affecting downhole fracture frequency include distance to fault, fault throw, lithology, and the presence of fault splays. Cores in this study are generally too far (>250 m) from major NNE/NE striking Taconic faults for fracture frequency to be affected by the main faults according studies by Agle et al. (2006) and O’Hara et al. (2015), even for faults with large throw. Five cores have % total organic carbon (TOC) and/or % sand data. Of the 17 clusters of high fracture frequency (fracture intensification domains, FIDs), 7 correlate with higher values of brittleness indices. A combination of the factors above and the possible presence of subsurface fault splays could explain the location of FIDs in the cores. Fracture clusters in 74NY10 at the Trenton-Utica contact were omitted because they may represent complications associated with karst collapse and subsequent sediment infilling. In cores with TOC data, possible beef structures correspond to higher TOC intervals, which could indicate overpressure from gas generation was approximately contemporaneous with calcite veining. These relationships provide a check of possible gas/vein ages when coupled with subsidence curves.