Crosswell seismic imaging and inversion of a michigan reef from above and from beneath

Summary: A high-resolution crosswell seismic survey was conducted using two monitor wells located external to a Silurian (Niagaran) reef, at Springdale field in northern Michigan. These wells extended to much greater depths than the reef, and imaging was conducted both from above and from beneath the reef. The resulting seismic images provide the best views of any Niagaran pinnacle reef obtained to date. The tops of the reservoir can be clearly distinguished, and its lateral extent or dipping edges can be observed along the profile. Reflection events internal to the reef are evident; some of them are fairly continuous across the reef and others are discontinuous. Inversion of the seismic data indicates which events represent zones of higher porosity and which are lower porosity or anhydrite-plugged. The full stacked images include angles that are beyond critical for many of the interfaces, and some reflections are visible only for a small range of angles, presumably near their critical angle. Strong apparent attenuation of signals occurs when seismic ray paths pass through the upper part of the Springdale reservoir; this may be due to intrinsic attenuation and/or scattering of events due to the locally strongly varying gas saturation and extremely low fluid pressures. The seismic images obtained, and interpretations of them, as assisted by inversion, provide additional insight into the internal geometry of this reef and provide data that should be useful for reservoir management