Delineation of a Mud Volcano Complex, Surficial Mudflows, Slump Blocks, and Shallow Gas Reservoirs, Offshore Azerbaijan
Authors: Corthay J.E. , Aliyev A.A. 
Published: October 2008

Seafloor and sub-seafloor interpretive maps derived from broad-band 3D Short-Offset seismic augmented with 2D High-Resolution seismic and seabed piston core data were utilized to assess potential hazards and constraints to mobile drilling operations over the Nakhchivan block, offshore Azerbaijan.

The Nakhchivan block is located along the shelf break and upper slope environment of the Azerbaijan sector of the South Caspian Sea. Water depth ranges from 70 to 788 meters across a northwest trending ridge that is the seafloor expression of a subsurface anticline. A flat-topped mud volcano complex and associated mudflow deposits occur along the anticlinal crest within a large depression bound on three sides by steep high-relief slump scarps and rotated blocks. The depression opens to the east into a large sea valley bound on both sides by slump scarps. Surface mudflows trend seaward through this valley on the eastern flank of the anticline. Gravity cores acquired within the mud volcano complex and surface mudflows recovered liquid to very soft black to grey muds. Geochemical analyses revealed anomalous concentrations of methane in all these cores.

A linear compressional anticline, trending from northwest-southwest to north-south, dominates the shallow structure. The axis of the structure, a complex fault graben, is truncated by a mud volcano. Immediately north of the mud volcano, a northwest to northeast trending radial fault pattern, aligned with the anticlinal axis, forms numerous narrow horst and graben structures. Four mapped horizons illustrate the subsurface structure and morphology. The latest Pleistocene Lower Khvalynian horizon is an unconformity that separates muds and minor sands above from interbedded clays, silty-clays, sands, and mudflow breccia below. The late Pleistocene intra-Lower Khazarian horizon is a transgressive surface that overlies a sequence of clays, thin sands, and mudflow breccia. The middle Pleistocene Apsheron horizon is an unconformity that overlies a sequence of thick clays, shales, and thin interbedded and basal channel-fill sands. The Akchagyl horizon, at the Plio-Pleistocene boundary, overlies thick clays, shales, interbedded and channel-fill sands, evaporites, and bioclastic carbonates. Sediments within the underlying upper Surakhany Series consist of interbedded shales, thin sands, and evaporites.

Rapid structural growth, associated seismicity and mud volcanism that was initiated at the onset of Apsheron time continues to the present. During early Apsheron time mud volcanism exploited the faulted axial crest of the anticline to form a mud lake and associated mud cones at the seafloor. Subsequent periods of mud volcano eruption, fault movement, and seismicity precipitated the formation of large slump blocks and surface slides. These processes formed the seafloor caldera observed today. Seafloor mudflows, active slumps and slides represent potential foundation hazards.

Structural and stratigraphic gas traps occur along the updip flanks of the anticline throughout the latest Tertiary and Quaternary section. Overpressured aquifers associated with interbedded sands and evaporites occur within the Akchagyl and Surakhany Series. Shallow overpressured water and gas sands represent a potential well-control hazard.

Introduction

The interpretive results of a mobile drilling rig site investigation on the SOCAR and ExxonMobil Nakhchivan exploration block are presented.