Velocity anomalies and out-of-plane reflections in Barnicarndy 1, Canning Basin

Abstract

Barnicarndy 1 was drilled as a stratigraphic well and penetrated 2585m of clastic sedimentary rocks overlying an angular unconformity composed of Proterozoic dolomite basement near SSE end of the eponymous graben (previously known as the Waukarlycarly Embayment) of the Canning Basin. The post-drilling data exhibited several types of anomalies and inconsistencies, including ultra-slow velocity of the vertical seismic profile (VSP) within the top 210m, substantially high velocity from 210 to 1390m, poor synthetic–seismic correlation, and discrepancies between the prognosis and actual depths. The VSP shows that the uppermost sedimentary section has an average velocity of less than 1000m/s, which is unrealistically slow and probably caused by a systematic time delay in the data acquisition. Such a delaying effect was removed when calculating the VSP interval velocity, which had reasonable consistencies with sonic velocity and lithological packages. The consistencies verify that the sections from 210 to 1390m have high velocities that are beyond the range of offset wells at equivalent depths and do not follow an expected trend along most of the well trajectory. Quantitative analyses of multiple synthetic correlations suggest that the depth discrepancies in the prognosis are not related to pre-drilling seismic interpretation, as the two-way time picks for major boundaries mostly match what has been penetrated in Barnicarndy 1. Two thirds of the depth error for basement prognosis comes from the unexpected high velocity in the shallow formations. The velocity anomalies indicate that the Barnicarndy Graben possibly has a complex history of tectonic movement, and thick sections could have been eroded during the Late Triassic. The remaining discrepancy in depth prognosis for the basement is related to the out-of-plane seismic reflection near the SSE end of the graben. The steeply dipping basement causes out-of-plane issues for 2D seismic data, and the Kidson seismic survey images the shallower basement reflected from further south, rather than what is vertically below the survey route.