USING TOPOLOGY TO CHARACTERIZE FRACTURE NETWORK IN A NORMAL FAULT SPLAY ZONE IN THE RIO GRANDE RIFT, NEW MEXICO

Geometry of fracture networks is a key element in reservoir characterization. Here we use topology indices to delineate fracture networks within the Plaza Blanca fault splay zone. The Plaza Blanca fault is located 4.65 km NE of Abiquiu in north-central New Mexico. This normal fault is formed during Rio Grande rifting. Its master fault strikes N75E. Eastward, the fault splays into two SE-dipping, synthetic normal faults: a N15E-striking fault in the north and a N70E-striking fault in the south. Abundant subsidiary faults are developed in the splay zone. Each fault juxtaposes the Oligocene-Miocene rift-fill Abiquiu Formation in both the hanging wall and footwall. Stratigraphic correlations reveal that the northern and southern splay have a throw of at least 20 m and 30 m, respectively.

To characterize the fracture network we adopt the circular window method to avoid sampling orientation bias. We restrict sampling to meter-thick, medium-grained, massive sandstone beds to eliminate the influences of bed thickness and lithology on fracture behavior. To acquire the topology feature, we count the number of 3 types of nodes within each circle: isolated tips (I-nodes), abutting fractures (Y-nodes), and crossing fractures (X-nodes). The percentage of I-, Y-, and X-nodes is indicative to the fracture connectivity. Higher I-nodes percentage means more isolated fractures. On the contrary, greater amount of Y-nodes and X-nodes suggests a higher connectivity.

Our preliminary data from 20 circular windows on sub-horizontal surfaces shows that fracture connectivity strongly correlates with structural position. Background fractures generally have little connections (I = 30-70%, Y = 30-70%, X = 0%); those from the outer part of the damage zone show an increased connectivity (I = 25-40%, Y = 55-75%, X = 0-5%). Fractures are highly linked within the splay zone with elevated proportion of Y- and X-nodes (I = X = 8-16%); and the highest connectivity is found at the splay point (I = 8-9%, Y = 55-56%, X = 35-36%). Fracture connectivity is also positive correlated with fracture frequency and intensity across the fault splay zone. Further work includes fracture analysis on vertical surfaces of the Plaza Blanca fault splay based on high-resolution images, in order to better understand and visualize fracture connectivity in a normal fault splay.