AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Source Rock Prediction from Seismic Part I: Links Between Rock Properties and Seismic Attributes
(1) Statoil ASA, Trondheim, Norway.
(2) Statoil ASA, Stavanger, Norway.
Presence of source rock is the first prerequisite for a hydrocarbon accumulation to occur. The purpose of this paper is to demonstrate why conventional seismic data can be used to risk source rock presence in petroleum system analyses by establishing a link between the smallest building blocks of a source rock to its seismic expression.
Published laboratory measurements show that increasing organic contents in shales reduce their velocities and density and increase their elastic anisotropy. Wireline logs combined with sample measurements are regularly used to assess the presence of organic-rich intervals from well data. Few studies however, have verified to what extent the relationships between organic content and acoustic-elastic rock properties determined in an exploration well can be observed on seismic data.
The relationship between organic content and rock properties of shale source rocks has been studied in fully cored scientific boreholes and in numerous exploration wells. The results show that bulk density (Rhob) is linearly reduced, while the compression velocity (Vp) and shear velocity (Vs) are non-linearly reduced with increasing organic content. As a consequence, acoustic impedance (Zp) is reduced non-linearly while the Vp/Vs-ratio increases with increasing organic content.
Forward modelling of reflectivity and amplitude versus offset (AVO) behaviour suggests that the top of a source rock interval on zero-phase data with normal polarity is characterized by a drop in impedance that produces a negative reflection coefficient or a ‘soft’ response. This negative normal incident reflection decreases from near to far offset, i.e., being a class 4 AVO response. The seismic responses at the top and base of source rock intervals will depend upon factors such as layer thickness and variations in richness (organic profiles). Our models suggest that source rock shale intervals can be identified on seismic data if their total organic content (TOC) is larger than 3-4% and their thickness is more than 15m. We conclude that rock property analyses of organic-rich shales demonstrate that they have characteristic acoustic properties that allow for robust seismic interpretation.