MIOCENE DEPOSITION AND PETROLEUM GEOLOGY— NORTHERN GULF OF MEXICO

Based on a workshop sponsored by PTTC's Eastern Gulf Region, November 10, 1999 in Jackson, Mississippi.

BOTTOM LINE

Exploration geoscientists working in the northern Gulf of Mexico and adjacent areas must develop insights into relationships between hydrocarbon occurrences and regional/ global events such as growth-fault/ allochthonous salt evolution and sea-level/ climatic cycles.

PROBLEM ADDRESSED

In the Gulf of Mexico, numerous hydrocarbon plays have been spawned by Mioceneage high frequency cycles of fluvial/ incised valley fill, deltaic/ neritic, and deep water slope/ submarine fans deposits. Many of these have been complicated by structural movements including salt diapirism. Over past decades, technological advancement has fueled development of exploration trends to greater depths and into deeper water. These areas include the Deep Water play, the Eastern Gulf of Mexico Unconfined Turbidite play, the Flex Trend, the Onshore South Louisiana play, and a "sub-weld" play in South Louisiana beneath the horizontal detachment surface.

KEY WORDS:

Deep Water Play, Eastern Gulf Turbidite, Flex Trend Play, Gulf of Mexico, Miocene, Reflection Seismic, Submarine Fan

SPEAKERS

Miocene--A Global Perspective; Lower & Middle Miocene Sequences and Depocenters in the Gulf of Mexico
Richard Fillon, Earth Studies Associates

Upper Miocene Sequences and Depocenters; Reservoir and Exploration Strategies in the Miocene of the Gulf of Mexico
Paul Lawless, CNG Producing Co.

TECHNOLOGY OVERVIEW

The Miocene is a pivotal interval in the history of the Cenozoic. Within its nearly 19 million years, profound oceanographic and climatic changes occurred. These include the transition from globally more uniform environments of the Paleogene, to the modern world where extreme climatic and oceanographic contrasts are the norm. Important Miocene climatic changes are reflected by the increasing importance of higher frequency cycles of deposition in the Gulf of Mexico.

Miocene Exploration History in the Gulf of Mexico. Historically in the Gulf of Mexico, exploration plays have been categorized into geographic areas such as onshore, flex trend, eastern Gulf of Mexico, and deep water. Technical modifiers such as amplitude, nonamplitude, and sub-salt have also been added. Geophysical, geologic, paleontologic and drilling technologies have all played an important role at one time or another.

In the onshore Miocene fluvial and deltaic trends of south Louisiana, gravity and sub-surface geological map ping found huge salt dome and faulted anticlinal fields early in the century. The addition of reflection seismic data after World War II helped extend deeper plays onshore— peaking in the 1940s for the upper Miocene, in the 1950s for the middle Miocene, and in the 1970s for the lower Miocene deltaic trends.

Deeper drilling technology pushed the search for upper Miocene deltaic plays onto the continental shelf. A major change in technology occurred in the early 1970s with the advancement of true amplitude processing of reflection seismic data. "Bright Spot" exploration strategy launched a brief era of "pure" geophysical prospecting. Amplitude-related plays were chased all over the shelf and into the uppermost deep-water areas of the northern Gulf continental slope in the 1980s. 3-D seismic data became commonplace on the shelf and upper-slope during this time. By the 1990s discoveries on the shelf had slowed, while deep-water discoveries began to take off.

Seismic advancements during the past two decades include dip moveout, advanced post-stack time migration algorithms, turning wave migration, 3-D amplitude versus offset processing, ray-trace analysis, post-stack depth migration, pre-stack time migration, and pre-stack depth migration. Also, industry has been willing to acquire 3-D data sets for production as well as exploration tools.

Introduction of "seismic stratigraphy" in 1977 drove the development of basin-fill and new submarine fan facies models. Major differences have been recognized between submarine fan sections in third-and fourth-order sequences deposited on a second-order relative fall of sea level, as opposed to submarine fan sections deposited on a second-order rise. Advances in biostratigraphy in the past two decades have greatly improved zonations and have allowed sequence stratigraphy to develop as an effective exploration tool. Modern computer technology has breathed new life into old exploratory field techniques such as gravity. Computer generated second-vertical derivative (SVD) gravity maps can now contrast low density salt with higher density sediment-filled minibasins, providing a high-resolution virtual image of shelf and slope structures.

Deeper Water Miocene. This play spans Mississippi Canyon, Ewing Bank, Atwater Valley Lund, and eastern portions of Green Canyon and Walker ridge where upper, middle and sometimes lower Miocene sections are economically drillable. Although oil is the main objective, large gas accumulations have also been located. This play has evolved from drilling to structural highs in the early 1980s to seeking amplitude-associated pay trapped in ponded turbidite facies in supra-salt and intra-salt mini-basins of the uppermost continental shelf in the late 1980s. Larger prospects in this area drilled in the 1990s have targeted salt overhangs and sub-salt structures. The recent billion barrel "Crazy Horse" discovery in southern Mississippi Canyon proved that large amounts of nonamplitude pay exist.

Basinward of the ponded turbidite mini-basin play, saltcored thrust folds have middle and lower Miocene abyssal plain section draped across them. Two discoveries occur in the Mississippi fan thrust and fold belt play, Neptune and Mad Dog.

Eastern Gulf of Mexico Unconfined Turbidite. This play targets upper and middle Miocene unconfined submarine fan section deposited basinward of the salt minibasin province in Main Pass and Vioska Knoll. Reservoirs in this trend lie within the channel-levee and overbank mid-fan depositonal system, and range in size up to 500 MMBOE.

Flex Trend. This trend is a geographic area encompassing the outermost shelf and uppermost slope where salt sheets are actively deforming. Discoveries from the 1970s and 1980s include "Bullwinkle" and "Popeye". In recent years the more exciting play has been for ponded upper Miocene, and possibly middle Miocene submarine fan section below deformed allochthonous salt. So far the play has been for amplitude-associated pay only and has made several major discoveries, but few economic fields. Salt sheets in the flex trend are as difficult to image beneath as the ones in deep water, thus this play has tended to be uneconomic. Targeting more homogeneous ponded sheet sands at the base of the sequence may improve the play's economics.

Onshore South Louisiana. Although the Miocene of south Louisiana has been heavily explored since the 1920s, it has provided more economic discoveries in the past ten years than the flex trend sub-salt play. Recent Miocene discoveries are deep, geopressured and contain reservoirs that are not amplitude associated. The largest discoveries in the play have been on faulted anticlines, found with the aid of detailed regional maps utilizing well logs, biostratigraphy, and 2-D seismic data. 3-D data were often acquired only after a discovery, for more efficient development.

Because it is so deeply buried, the exploration target for the Lentic Jeff submarine fan section is only 18 miles wide. In southeast Louisiana, a number of lower and middle Miocene turbidite sands, deposited on a relative second-order sea level rise have been located downdip of their respective paleo-shelf edges. Three additional middle Miocene third-order sequences contain very sandrich submarine fan sections. These sequences can be explored over a 25-50 mile wide swath.

Several large exploration companies have recently shut down their onshore exploration programs. Additional 3-D seismic (or at least rigorous reworking of the large amount of 2-D data in that area) may help to identify more economic stratigraphic traps. Many additional submarine fan prospects remain to be tested in this play. A "sub-weld" play exists in various parts of South Louisiana where section below the horizontal detachment surface can be drilled.

CONNECTIONS:

Richard H. Fillon
Earth Studies Associates
3730 Rue Nichole,
New Orleans, LA 70131
Phone 504-394-0797, fax: 425-955-4725 E-mail fillorh@bellsouth.net

Paul N. Lawless
CNG Producing Co.
1450 Poydras St.,
New Orleans, LA 70110
Phone 504-593-7000, fax: 504-593-7342, E-mail paul_n_lawless@cngpcng.com

For information on PTTC’s Eastern Gulf region and its activities contact:
Ernest A. Mancini, Professor of Geology University of Alabama
Box 870338, 202 Bevill Bldg., Tuscaloosa, AL 35487
Phone: 205-348-4319, Fax 205-348-0818, E-mail emancini@wgs.geo.ua.edu

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