Simple Method of Determining Sand-Shale Ratios from Seismic Analysis of Growth Faults: An Example from Late Oligocene to Early Miocene Niger Delta Deposits

Pochat, Stéphane¹, Jean Van Den Driessche² (1) Laboratoire de Planétologie et Géodynamique UMR-CNRS 6112 Université de Nantes, Faculté des Sciences et des Techniques, BP 92208 44322 Nantes Cedex 3, France (2) Géosciences Rennes, UMR 6118, Université de Rennes, 35042 Rennes, France

The T-Z plot method (throw T versus depth Z) is a simple geometric tool that graphical­ly represents strata thickness variations in growth-fault and growth–fold settings, and that has been used to infer fault kinematics. The slope variations of such a plot reflect variations in the degree of thickening of the strata towards the hanging-wall. If sedimentation always fill fault induced topography up to the top, the T-Z plot can be used to constrain the displace­ment history of growth faults. Thickening of strata towards the hanging-wall indicates a peri­od of fault growth, while non-thickened intervals are symptomatic of periods of tectonic qui­escence. Therefore, the assumption of fill-to-the-top sedimentation is not always justified, denies the existence of fault scarp and precludes directly determining fault kinematics from T-Z plots. Indeed in most depositional settings, sedimentation is characterized by the alter­nation between more and less energetically supplied materials. In the case of a continuous fault movement, decantation of shale particles makes sedimentation homogeneously distrib­uted across faults (un-thickened deposit) whereas dynamic deposition of sands fills the preferentially lows before (thickened deposit). The T-Z plot method allows to easily depict un-thickened and thickened sedimentary intervals from even rough seismic records and can therefore be used to predict sand-shale ratios. The method is here applied to a growth fault in the Niger delta that affects Oligocene to Early Miocene deltaic deposits. Most shale inter­vals are identified, and the sand-shale ratios are predicted. We suggest that the method can be a valuable tool in oil exploration.