FAULT EFFECTS IN THE UTICA BASED ON CORE FROM THE MOHAWK VALLEY REGION, NYS

Structure data from six unoriented cores in the Ordovician Utica black shale in central New York are used to determine the effect lithology, distance from “Taconic” NNE/NE-striking faults, and their throw may have on fracture intensification domains (FIDs) and structure characteristics, including vein-filled fracture length, aperture, frequency, slickenside/fiber orientation.

Cores 75-NY-2, 74-NY-12, and 75-NY-11 were previously studied in detail by members of the UB Rock Fracture Group. We completed a detailed structural analysis of cores 74-NY-10, 74-NY-9, and 74-NY-5. The Utica section contained 73 vein-filled fractures in 74-NY-10; 135 in 74-NY-9; and 12 in 74-NY-5. Two formerly-lost core boxes of 74-NY-10 revealed a damage zone at the Utica-Trenton contact that exhibits complex faulting (including a possible soft-sediment phase) and multi-phase vein development.

core #	Fault Name	core dist. to fault (km)	throw (m)	Dist. from thrust front (km)	hi side/lo side	FID (frac /m)	Avg. frac freq* (frac/m)	Utica Recovered in Core (m)	Fault Orientation
75-NY-2	Saratoga-McGregor	0	137	8	hi	28	3.31	137.4	NNE
74-NY-12	Hoffmans	0.55	381	23	hi	13	7.46	53.6	NNE
74-NY-10	E. Stone Arabia	1.46	24	58	lo	20	0.88	83.2	NE
75-NY-11	Hoffmans	1.73	381	23	lo	59	23.2	6.0	NNE
74-NY-9	Fonda	~2	>40	45	lo	21	1.33	101.5	NE
74-NY-5	Mother Creek	3.97	137	68	hi	4	0.05	220.4	NNE

*downhole fracture frequency

Primary factors that affect fracture frequency in the cores include distance to a fault, presence of fault splays, and lithology. All the cores have long intervals with no fractures (6.1-33.5 m). FIDs do not consistently correspond to increased silt/sand or carbonate content (7 of 12 do not correlate with more brittle lithology).

O’Hara found a positive relationship between the magnitude of fault slip and FID width. Based on this relationship, most cores are too far from the faults to display significant fracture effects from the main fault, consistent with earlier field observations in the Utica Shale by Agle and Jacobi. Fault splays and other unmapped faults with small throw likely account for the anomalous FIDs observed in core. Although Jacobi proposed possible strike slip motion along the faults based on 3D seismic and geologic map patterns, only cores associated with Hoffmans and Fonda faults display kinematic indicators consistent with oblique-slip.