Developments in Petroleum Science, 4 GEOMORPHOLOGY OF OIL AND GAS FIELDS IN SANDSTONE BODIES C.E.B. CONYBEARE Australian National University, Canberra (A.C . T.) ELSEVIER SCIENTIFIC PUBLISHING COMPANY Amsterdam - Oxford - New York 1976 ELSEVIER SCIENTIFIC PUBLISHING COMPANY 335 Jan van Galenstraat P.O. Box 211, Amsterdam, The Netherlands AMERICAN ELSEVIER PUBLISHING COMPANY, INC. 52 Vanderbilt Avenue New York, New York 10017 Library 01 C0ngrr.a Calaloging in Publication Data Conyheare, C E B Geomorphology of oil and gas fields in sandstone bodies. (Developments in petroleum science ; 4) Includes hibliographical references and indexes. 1. Geornorphology. 2. Rock traps (Hydraulic engineering) 3. Sandstone. 4. Petroleum-Geology. 5. Gas, Natural-Geology. I. Title. 11. Series. GB406. c63 553' .B 7537974 ISBN O-~!+I&-lJ.398-7 Copyright b 1976 by Elsevier Scientific Publishing Company, Amsterdam AH rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher, Elsevier Scientific Publishing Company, Jan van Galenstraat 335, Amsterdam Printed in The Netherlands ACKNOWLEDGEMENTS The author wishes to acknowledge the assistance of Ms P.M. Carle, Ms E.A. Kilner, Mr L. Seeuwen and Mr G.R. Harper in the preparation of the typescript and illustrations. Acknowledgement is also due to the following organizations, institutions, and publishers from whose publications the quota- tions and many of the illustrations in this book have been drawn: Geological Society of America, American Association of Petroleum Geologists, Society of Economic Paleontologists and Mineralogists, Canadian Society of Petroleum Geologists, Canadian Institute of Mining and Metallurgy, Royal Geological and Mining Society of the Netherlands, Australian Petroleum Exploration Association, Four Corners Geological Society, Delaware Geological Society, Rocky Mountains Association of Geologists, Wyoming Geological Association, Houston Geological Society, Gulf Coast Association of Geological Societies, Dallas Geological Society, Tulsa Geological Society, Kansas Geological Society- United States Geological Survey, U.S. Bureau of Reclamation, U.S. Navy Hydrographic Office, Mississippi River Commission, Illinois Geological Survey, Indiana Geologcal Survey, Geologische Dienst der Nederlanden, U.S.S.R. Ministry of Oil Industries, University of Texas, Schlumberger Well Services, Nedra Press (U.S.S.R.), John Wiley and Sons, Prentice-H'dl, Princeton University Press, Chapman and Hall, Springer-Verlag, Gulf Publishing Company, Tracer Petroleum and Mining Publications, and Elsevier Scientific Publishing Company. The author also wishes to acknowledge the facilities offered to him during visiting appointments at the University of Calgary and the Colorado School of Mines. PREFACE This book is essentially about stratigraphic traps for oil and gas. Many of the examples discussed are geomorphologic features having inherent closures without any secondary structural element; others are primarily geomorpho- logic features modified by folding or faulting to produce local closures. The first category comprises traps that are purely stratigraphic, although the accu- mulation of hydrocarbons may have been assisted by regional or local tilting of the strata, or by deformation caused by compaction of the underlying sedi- ments. The second category, which includes a much larger number of known examples, comprises structural-stratigraphic traps. Many of these traps have proved to be elusive, particularly those of the first category which commonly defy detection by seismic methods. In some cases, discovery has been ac- cidental, and further exploration to delineate the accumulation has been empirical. The purport of this book is to briefly present examples illustrating the main geological characteristics of geomorphologic features that have controlled or influenced the accumulation of oil and gas in particular fields, with a view to using such examples as models in the search for new fields in sandstone bodies. Many of the examples presented have been so well documented that they stand as classic examples of stratigraphic fields in which oil and gas accumula- tions are controlled by geomorphologic features. Others have yet to be defined unequivocally, but are included as additional references to assist in the inter- pretation of geophysical and sub-surface geological data. The author is indebted to the many geologists who have written about the hydrocarbon accumulations and geological features described herein, without whose efforts it would not have been possible to compile this book. Canberra, A.C.T. C.E.B. CONYBEARE 1 INTRODUCTION Accumulation of oil and gas in a sandstone body depends on several factors including the state of generation and time of migration of hydrocarbons or their precursors, directional variations in porosity and permeability, the existence of stratigraphic or structural closure with a suitable seal, and the geometry of the sandstone body. Many holes have been drilled on the basis of geophysical inteqretations that indicated structural closure within a prospective section, only to find the section lacking in suitable source beds for hydrocarbons, or with no impermeable seal above the potential sandstone reservoir. The sandstone itself may be locally tight. Further, the spatial relation- ships of depositional trends and geometry to permeable zones within the sandstone body are commonly unknown. To complicate our understanding of the situation, the depositional trends and geometry of the sandstone body itself may not be known. With these possibilities in mind, the following comments are offered on the classification of sandstone bodies. A sheet or blanket sandstone body may be designated as a mappable stratigraphic unit, such as a member or formation, and yet lack continuity and homogeneity. At one locality it may consist of a single sandstone unit, and at another it may comprise two or more sandstone beds that have individual depositional trends, shapes, and petrophysical characteristics. At a particular location oil or gas may be encountered in Sandstone "A", where it occurs below the up-dip edge, but not in adjacent Sandstone "B" that pinches out elsewhere. This type of situation is common in alluvial point bar and channel-fill sands, in anastomosing delta distributary sands, and in off-lapoing marine shoreline sands. 2 A classification of sand body shapes is proposed by Pettijohn, Potter and Siever (1972) after the classification of Potter (1962, b). They say that there are at least four different basic recurring shapes to sand bodies, illustrated by Fig. 1-1, and make the following statement on p. 440, "Equidimensional sand bodies have length-width ratios of approximately 1:l and may cover a few to thousands of square kilometers. These have been called sheets and blankets. Elongate sand bodies, on the other hand, are those with long dimension notably exceeding width and are one of three types: pods, ribbons and dendroids (Potter, 1962, Fig. 3). -have length-width ratios of three or less where ribbons are much more elongate with length-width ratios of three or more and possibly as high as 20 to 1 or more. Rich (1923, p. 103) used the term shoestring for such bodies. Dendroids are commonly more sinuous and have branches, either tributaries or distributaries. By lateral migration, coalescent ribbons and dendroids may form belts, dendritic belts being the more common. 'I Sheets E Ion gate Fig. 1-1 Classification of sand body shapes. (Modified by Pettijohn, Potter and Siever, 1972, after Potter, 1962b). 3 These designations are based on the geometry of the sandstone bodies and do not have any implicit connotation as to depositional environment or geomorphology. Also, they can be misconstrued and misapplied, with particular reference to some so-called sheet or blanket sandstones. Nevertheless, they serve a useful purpose in qualifying and to some extent quantifying the shapes of sandstone bodies. Sheet-like stratigraphic units, consisting essentially of sandstone, may have originated as transgressive or regressive shoreline sands, as eolian sands, as widespread sand beds within coalescing alluvial fans, as braided and laterally migrating estuarine deposits, as river sediments on a broad plain, or as layers of sand swept out on abyssal plains of the ocean. Apart from the similarity of their gross dimensions, these units are markedly different in their internal structure and stratigraphic relationships. All are diachronous to some degree, although a layer of sand swept rapidly on to an abyssal plain will represent so short a period of time that it can be regarded as a stratigraphic marker bed. Internally, a sheet-like stratigraphic unit may consist of several distinct sandstone bodies that may be locally connected or entirely separated by impermeable shale layers. These separate bodies may be nearly equidimensional or elongate in shape. A sequence of off-lapping, elongate shoreline sands may have a wide areal distribution within a comparatively thin stratigraphic interval, and consequently form a sheet- -like unit in gross dimensions. Within such an interval the preferred orientations of these elongate sandstone bodies are parallel to the original coastline; but the interval may also include other elongate sandstone bodies, oriented approximately normal to the coastline, that were formed as distributary sands filling channels cut into the shoreline sands. In many cases the distributary sands cannot readily be distinguished from the shoreline sands with which they are associated, although variations 4 ENVIRONM E NTS CHANNELS STREAM SHEET F LOODS ALLUVIAL FLOWS 4 FANS “SIEVE DEPOSITS” (APEX, MIDDLE I DEBRIS FLOWS 8 BASE OF FAN) VlSCOUS 4 FLOWS MUD FLOWS CHANNELS (VARYING SIZES) BRA I DED ALLUVIAL STREAMS ,,, LONGlTUDlNAl ,x (FLUVIAL) Q TRANSVERSE CHANNELS MEANDER MEANDERING NATURAL LEVEES BELTS STREAMS (ALLUVIAL POlNT BARS VALLEY) 1 STREAMS, LAKES F LOODBASINS 8 SWAMPS 1 TYPES: COASTAL DUNES TRANSVERSE SElF EOLIAN DUNES DESERT DUNES ( LONGlTUDlN AL ) BARCHAN PARABOLIC OTHER DUNES DOME-SHAPED Figs. 1-2, 1-3 and 1-4 Classification of depositional environments of sand bodies and their related geomorphologic features. (After Le Blanc, 1972, and Bernard and Le Blanc, 1965). 5 DEPOSIT IONA L MODELS ORAlDtD CHANNtLS IWAIHtll AND AOANDONfD CMANNfL5 ALLUVIAL FAN UfN, MEANDERING STREAM COASTAL DUNES 6 E NVI RO N M E NTS ~~ CHANNELS MEANDER NATURAL LEVEES BELTS UPPER DELTAll POINT BARS PLAIN ST REAMS, F LOODB A S INS LAKES & SWAMPS CHANN EL5 DlSTR IBUTARY 2a C HANN ELS NATURAL LEVEE! Z 0- c- LOWER DELTAIC DELTAIC v, Z PLAIN MARSH, a INTER- LAKES, DISTRIBUTARY pi TIDAL CHANNEL: I- AREAS & TIDAL FLATS R IV ER-MOUTH BARS - INNER BEACHES & ?: F RlNGE 1 BEACH RIDGES L L TIDAL FLATS DISTAL Fig. 1-3. For caption see p.4.