Dip Sequence Analysis Utilizing Statistical Curvature Analysis: A Case Study on Complex Strata of the Late Middle Miocene Formation in Akita Basin, Onshore Japan

Abstract

Variation of dips from point to point in the subsurface is related to bulk curvature of the structural setting. Conventionally, consistent shale bedding dips or low magnitude consistent bedding are selected and filtered out to interpret only the transverse section of the wellbore and define the subsurface dip sequences obtained from the microresistivity images or dipmeter data. In this study, complex lithologic characters and associated structures create uncertainty in shale identification, making it difficult to analyze the structural dip pattern. Bengtson’s (1980) Statistical Curvature Analysis Technique (SCAT) was adopted to analyze and identify key components of the structural geology. Bedding dips were statistically analyzed in detail and revealed three possible distinct structural zones. Zone 1 shows a consistent average dipping azimuth toward the east, with increasing dip magnitude from the top to base. Zone 2 exhibits the same azimuth, however, with the dip magnitude decreasing from approximately 70 degrees to less than 10 degrees. The most significant change occurs in zone 3 where the dipping orientation changed drastically. the dip-azimuth plot revealed the trends of beddings, mainly toward the south at the lower part and dipping toward the east toward the upper part. The dip trends in the longitudinal dip component plot with depth in the study well exhibits considerable scatter that may be attributable to the steep dips zone. The transverse dip component exhibits the distinctive negative cusp pattern. The dips exhibit the downward increase in magnitude until they reach the drag zone, then return to the normal dip magnitude trend. In the present well, the dips increase from 10 degrees to approximately 70 degrees, and are greater between the trough and crestal plane areas. The SCAT analysis detected the presence of a sub-seismic plunging normal fault crossing the drilled section. This normal fault has a flattening drag that plunges at 8 degrees across the longitudinal direction of approximately 160 degrees (strike). This model may be used for the constraints of structural interpretation where seismic data is inconclusive. It was beneficial in revealing subsurface structural geometry. This helps to decipher the sub-seismic structural geometry of the target structure in high resolution, provide more detailed structural information, and visualize possible different structural zonation in the drilled section.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018