Advances in Organic Geochemistry 1985 Org. Geochem. Vol. 10, pp. 377-389, 1986 Printed in Great Britain. All rights reserved 0146-6380/86 $3.00 + 0.00 Copyright © 1986 Pergamon Journals Ltd Stable carbon isotopes and biomarkers as tools in understanding genetic relationship, maturation, biodegradation, and migration of crude oils in the Northern Peruvian Oriente (Maranon) Basin ZvI SOFER j, JOHN E. ZUMBERGE 1. and VICTOR LAY2 ~Cities Service Oil and Gas Corporation, Exploration and Production Group, Applied Research and Technology, P.O. Box 3908, Tulsa, OK 74102, U.S.A. 2Occidental Petroleum Corporation of Peru, Los Nardos 1018, San Isidro, Lima 27, Peru (Received 12 September 1985; accepted 6 March 1986) Abstract--The geochemistry of 15 oils produced from the Cretaceous Vivian and Chonta formations in the Northern Peruvian Oriente (Maranon) Basin was studied. This study demonstrates the usefulness of both stable carbon isotopes and biomarkers in understanding genetic relationships (source), maturation, biodegradation, and migration of oils. Specifically, using stable carbon isotopes and statistical factor analysis, two groups of genetically different oils were identified in the northern Oriente (Maranon) Basin. The Chonta Formation is the reservoir for one group while the other is exclusively contained in the Vivian Formation. Both oil families seem to be derived from a marine organic source which had some terrigenous organic input. The presence of demethylated hopanes in some of the Vivian oils indicates that these oils have been severely biodegraded in the past by meteoric water influx. Breeching of the Vivian Formation to the north, shortly after accumulation at the Cretaceous-Tertiary boundary, allowed the penetration of the meteoric water. Biodegradation stopped in early Tertiary when the Vivian unconformity was covered with impermeable red beds. Following this, a secondary pulse of nondegraded Vivian-type oil has reached some of the biodegraded Vivian reservoirs. The secondary pulse was identified by the presence of n-paraffins in the biodegraded oils, and stable carbon isotopes aided in identifying the source of this later oil. This secondary pulse may have been triggered by a southern tilting in Eocene time or by the differential subsidence of the basin that resulted from the rising of the Andean Mountains (with major movements in Miocene time). Using this information, it was concluded that the distribution of biodegraded oils in the Vivian Formation reflects the extent to which fresh water has penetrated the subsurface. The distribution of reservoirs with a secondary pulse was controlled by the availability of reservoirs filled with nondegraded oil in the direction from which the secondary migration originated, the size of the pulse, and the distance it has migrated. Variations in the salinity of the Vivian Formation waters are likely the result of replacement of the meteoric waters (that caused biodegradation) by highly saline formation waters originating in deeper parts of the basin. Using steranes and triaromatic steranes, it was found that the Chonta and Vivian oils were generated from mature source rocks (triaromatic steranes indicate that the maturity of the source was equivalent to vitrinite reflectance of approx. 1.0-1.35%). Because of the low maturity (and low organic content) of rocks in the neighborhood of the reservoirs, it was concluded that the oil was generated and migrated most probably from the west in what is now the Andean Mountains. Because of the time constraints put by the biodegradation event in the Vivian Formation, most of the migration of the early oils must have been completed by late Cretaceous. During the time span required for the first migration, the maturity of the source rocks of the oils increased from vitrinite reflectance 1.0 to -1.35%. The mode of accumulation in the basin was generally from west to east with the more recent and more mature oil replacing the earlier and less mature oil in the reservoir. The earlier oil continued then to migrate to the east, accumulating in the next reservoir. Key words: stable carbon isotopes, biomarkers, oil-oil correlation, maturation, biodegradation, migration, oriente, Maranon, Peru, factor analysis INTRODUCTION The Northern Peruvian Oriente (Maranon) basin is a broad polihistory, pericratonic basin covering an area of about 49.4 million acres in the eastern portion of Peru. The sedimentary fill reaches a maximum thick- ness of about 50,000 ft in the west and consists of rocks ranging in age from Ordovician to Quaternary. The basin at present is bounded to the west by the *Present address: Ruska Laboratories, P.O. Box 742688, Houston, TX 77274, U.S.A. Andean Cordillera and to the east by the Guainian shield. This configuration was delineated when the Andean geosyncline started uplifting during the Late Cretaceous and continued intermittently with major movements during Early Paleocene to Late Oligocene and Pliocene ages. The basin is bounded and crossed by several arches which have been related to Her- cynian and Eohercynian tectonic movements. Appar- ently, subsidence of the basin began when the Andean belt began significant uplift, although the most im- portant sinking and sedimentary filling occurred only after the Oligocene (35 Myr B.P.). 377 OG 10iI--Y