SEQUENCE STRATIGRAPHY OF NIGER DELTA, ROBERTKIRI FIELD, ONSHORE NIGERIA A Thesis by OLUSOLA AKINTAYO MAGBAGBEOLA Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE December 2005 Major Subject: Geology SEQUENCE STRATIGRAPHY OF NIGER DELTA, ROBERTKIRI FIELD, ONSHORE NIGERIA A Thesis by OLUSOLA AKINTAYO MAGBAGBEOLA Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Approved by: Chair of Committee, Brian J. Willis Committee Members, Steven L. Dorobek William. Bryant Head of Department, Richard F. Carlson December 2005 Major Subject: Geology iii ABSTRACT Sequence Stratigraphy of Niger Delta, Robertkiri Field, Onshore Nigeria. (December 2005) Olusola Akintayo Magbagbeola, B.Sc. (Honors); University of Ilorin, Ilorin, Nigeria; M.Sc., University of Ibadan, Ibadan, Nigeria Chair of Advisory Committee: Dr. Brian J. Willis Deposits of Robertkiri field, in the central offshore area of Niger Delta, comprise a 4 km thick succession of Pliocene to Miocene non-marine and shallow marine deposits. A sequence stratigraphic framework for Robertkiri field strata was constructed by combining data from 20 well logs and a seismic volume spanning 1400 km2. Major sequences, hundreds of meters thick, define layers of reservoir and sealing strata formed during episodic progradation and retrogradation of deltaic shorelines. These deposits progress upward from fine-grained prodelta and deep water shales of the Akata Formation through paralic sandstone-shale units of the Agbada Formation and finally to sandy non-marine deposits of the Benin Formation. The Agbada Formation is divided into six third-order sequences starting at the first seismic reflection that can be mapped across the seismic volume. The Agbada Formation under Robertkiri field is complexly deformed across a succession of major, cuspate, offshore-dipping, normal faults, and associated antithetic faults and rollover anticlines within down-dropped blocks. iv Thickening of intervals between some reflections across major faults and away from the crests of adjacent rollover anticlines suggest syndepositional displacement. Relationships between major faults and the thickness of transparent seismic facies that comprise lower parts of the seismic record suggest faulting was associated with movement of undercompacted shales within the Akata and lower Agbada Formations. Robertkiri field is located along the proximal margin of the Coastal Swamp I depobelt, a subbasin within the Niger Delta clastic wedge formed by margin collapse into underlying undercompacted shale. Accommodation and sequence development in this setting is controlled by both structural faulting and sea level fluctuations. Upsection, sequences become thinner, more laterally uniform in thickness, less structurally deformed and contain less growth strata. Erosion along sequence boundaries becomes progressively shallower and broader, as accommodation under Robertkiri field declined and more sediment was bypassed basinward. Incisions along the base of older sequences (>100 m) is greater than 3rd order sea level falls reported to occur during the Miocene, which suggests that there were local areas of tectonic uplift within this dominantly extensional setting. v DEDICATION I dedicate this thesis to God, my darling wife, OluwaToyin Rita Magbagbeola and son, OluwaLONImi (LONI). ‘Tayo (jr). Magbagbeola. vi ACKNOWLEDGMENTS Many thanks to Dr. Brian Willis, the chair of my thesis committee for his tireless contributions to the success of this study. I owe a debt of gratitude to Dr. Steve Dorobek and Dr. William Bryant for the assistance and contribution in reviewing the manuscript. I thank my employer, Chevron Nigeria Limited, for providing a scholarship and resources for this program. I am equally grateful for my manager in Nigeria, Mr. A. J. Akinwale, for his support and encouragement always. Thanks to ‘Dayo Adeogba and Monsur Akinyele for their wealth of knowledge and support at all times. Finally, I appreciate my wife OluwaToyin Rita Magbagbeola, son Loni Magbagbeola and parents Deacon and Deaconess E. Magbagbeola. vii TABLE OF CONTENTS Page ABSTRACT.................................................................................................................... iii DEDICATION ................................................................................................................ v ACKNOWLEDGMENTS............................................................................................... vi TABLE OF CONTENTS................................................................................................ vii LIST OF FIGURES......................................................................................................... ix LIST OF TABLES .......................................................................................................... xiv INTRODUCTION........................................................................................................... 1 THE NIGER DELTA...................................................................................................... 6 Regional Setting ............................................................................................... 6 Formations and Depositional Environments…................................................ 9 DATA AND METHODOLOGY .................................................................................... 16 Data .................................................................................................................. 16 Log Description................................................................................................ 16 Biostratigraphy ................................................................................................. 20 Seismic Volume ............................................................................................... 20 Sequence Stratigraphic Division ...................................................................... 21 STRUCTURE AND STRATIGRAPHY OF ROBERTKIRI FIELD ............................. 29 Structure ........................................................................................................... 29 Stratigraphy ..................................................................................................... 45 Interpretation of Stratigraphic Trends ............................................................. 73 DISCUSSION ................................................................................................................. 77 CONCLUSIONS............................................................................................................. 83 viii Page REFERENCES CITED ................................................................................................... 85 VITA ............................................................................................................................... 89 ix LIST OF FIGURES FIGURE Page 1 Location map of study area ........................................................................ 5 2 Depobelts with their relative ages striking northwest - southeast..................................................................................................... 7 3 Physiographic sketch of the deep marine sediments in the Gulf of Guinea offshore Niger Delta.................................................... 10 4 Schematic diagram of Niger Delta regional stratigraphy, and variable density seismic display of main stratigraphic units with corresponding reflections ..................................... 11 5 LANDSAT TM mosaic of mixed wave, tide and fluvial influenced Niger Delta .............................................................................. 15 6 Seismic survey of study area (upper polygon) showing thickness of deposits addressed in this study and traces of major faults ............................................................................................ 17 7a Example of well log through the Niger Delta in Robertkiri field showing overall upward coarsening trend and biostratigraphic zones, depositional environments and age estimated from unpublished Chevron report............................................................................................ 18 7b Representative gamma ray patterns observed in Robertkiri field (a) Fining upward (b) Coarsening upward (c) Symmetrical and (d) Blocky log patterns ................................ 19 8 Seismic sections along A A’ and B’ B’’ with well logs showing stratigraphic discontinuities, incisional features and general seismic facies character .......................................................... 22 9a Gamma ray correlation log panel for wells 02, 11, 07 and 03 indicating an overall upward weakly coarsening sequence ..................................................................................................... 25 9b Gamma ray correlation log panel for wells 02, 13 and 03 indicating an overall upward weakly coarsening sequence ..................................................................................................... 26 x FIGURE Page 9c Gamma ray correlation log panel for wells 02, 13 and 05 indicating an overall upward weakly coarsening sequence ..................................................................................................... 27 9d Gamma ray correlation log panel for wells 09, 13 and 11 indicating an overall upward