Automated Cyclostratigraphic Analysis in Carbonate Mudrocks Using Borehole Images

Abstract

Stacking analysis provides a useful method for correlation and sequence stratigraphic analysis. Deep-marine mudrocks have fine scale vertical heterogeneity that is commonly ignored in conventional core descriptions. With the move to exploit resources plays, quantifying this heterogeneity is essential. Borehole images are the most suitable dataset to use because of their high resolution (up to 0.2 inch vertical resolution). However, manual processing of data at this level of detail is time consuming and can be subjective. In addition, correlations between two adjacent wells can be difficult based solely on facies, due to the large number of cycles observed. In this study, we present a workflow for automated stacking analysis using the Modified Fischer Plot (MFP) algorithm from borehole images. MFP measure the departure from mean cycle thickness. Hundreds of feet can be analyzed in tens of seconds to minutes in an objective manner. Because of their relatively planer lamination, deep-marine sediments lend themselves to automation. Automated picking of intervals in borehole images is based on contrast detection in resistivity values. After a quick quality inspection, suitable pad images are cross-correlated and aligned. They are summed together to increase signal to noise ratio. Smoothing is used to remove irregularities. Vertical difference in the resultant data is calculated. A contrast limit is used to define surfaces. Obtained intervals are then used to calculate a MFP. We apply the workflow on a deep-water carbonate mudrock succession. A geologic model is used to define cycles in borehole images. The cycles can be procedurally implemented in the workflow using a set of rules such as interval thickness, minimum resistivity contrast, and average interval resistivity. Core description and photos are used to ground truth borehole images of facies. A sequence stratigraphic interpretation is also made for the studied interval. Spectral analysis shows that picked intervals are ordered. The identified sequence stratigraphic surfaces have a signature on the MFP. The MFP can be used as an aid for sequence stratigraphic analysis. In general, automated MFP is suitable for rapid stacking pattern analysis when coupled with a geological understanding of the area.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014