Amplitude Anomalies, Condensed Sections, And Sequence Boundaries, Green Canyon Area, Gulf Of Mexico

Abstract

ABSTRACT The Green Canyon 92 #1 well targeted a lower Pleistocene amplitude anomaly interpreted to represent a hydrocarbon bearing sand at the base of a ponded, intraslope fan system. The target anomaly was not caused by a hydrocarbon bearing sand, but rather occurs at the boundary between a high acoustic impedance silty shale over a low acoustic impedance clay rich condensed shale. The prediction of a fan system was correct. However, the seismic expressions of the various facies were not as expected. A low reflectivity zone above the target anomaly, expected to represent a shale sequence, actually contained abundant (247 net feet) sand. In the GC 92/136 area, acoustic impedance values for clay rich shales, silty shales and sands overlap considerably. Silty shales have higher acoustic impedance values than shale or sand. Strong negative reflection coefficients result when silty shales overlie the clay rich shales that often occur at or near sequence boundaries. The low acoustic impedance, clay rich shales typically exhibit high neutron porosity and abundant nannofossils, and are interpreted to represent condensed sections. Thus, in this area, condensed sections are â??soft' streaks rather than â??hard' streaks, and often serve as the major seismic mapping surfaces in the Plio-Pleistocene. INTRODUCTION Exploration efforts in the Gulf of Mexico are often directed toward prospects supported by seismic amplitude anomalies. Consequently, Iithologic and fluid prediction using seismic data is important for exploration risk reduction. This paper documents a case history from the Green Canyon area, in which seismic facies, thickness trends and reflectivity were used pre-drill to predict Iithology and fluid content. Although the well was a dry hole, valuable lessons were learned which could improve the success rate for future exploration drilling in the deep water Gulf of Mexico. In this example, seismic facies are put in the context of a sequence stratigraphic model in order to predict the presence of reservoir quality sand. Thickness trends are also used to predict sand, because the Green Canyon area was in a deep water slope and basinal environment during the Pleistocene, when deposition was dominated by sediment gravity flows. Sand therefore tends to be concentrated in paleobathymetric lows, which are inferred by sediment isopach thicks. Polarity and amplitude of the target reflection are used to predict acoustic impedance (velocity × density), which is calibrated to well data in order to predict Iithology and fluid type. In the Green Canyon area, Pleistocene sediments are unconsolidated, and sands generally have lower acoustic impedance than shales. This contrast is enhanced by the presence of hydrocarbons (particularly gas) within the pore space of sands. Hydrocarbon bearing sands, therefore, often result in strong reflection coefficients which are negative at the top of the sand and positive at the base. Below sea level. All of these shallow reserves on blocks GC 136 and GC 137 are associated with strong seismic amplitude anomalies. However, these accumulations were determined to be sub-economic, so the Green Canyon 92 #1 well was drilled in an attempt to find additional, deeper pay.