SUBSALT EXPLORATION Inching closer to predicting subsalt oil and gas prospectivity

Leonard Le Blanc Editor Gulf of Mexico subsalt exploration trend. Subsalt wells cost $8-20 million each, excluding pre-drilling geoscience expenses, and are time-consuming to drill and analyze. Producers funding the US Gulf of Mexico subsalt play want to see results - like where to lease and which salt structure to look under.

Leonard Le Blanc

Subsalt wells cost $8-20 million each, excluding pre-drilling geoscience expenses, and are time-consuming to drill and analyze. Producers funding the US Gulf of Mexico subsalt play want to see results - like where to lease and which salt structure to look under.

In fact, how to predict hydrocarbon incidence under the salt structures, before drilling preferably, has become almost as important as finding oil and gas. 3D seismic has been key to the pursuit, but while the data is shedding light on subsalt stratigraphy, pore space contents are still largely a mystery.

At this stage of subsalt prospecting - 33 wells drilled with five discoveries - each exploration group has a different geological model and evaluation strategy. The randomness of oil and gas distribution under the salt sheets is still painfully apparent.

Besides pushing up the finding ratio for subsalt prospecting, there are related questions that subsalt exploration groups hope to begin answering before the end of 1996:

  • Creaming advantage? Can the present group of subsalt explorers "cream off" the best and largest reserve accumulations, or are the dry hole risks, an insufficient number of exploration wells, and high costs associated with 3D technology and drilling too great to assign a reasonable advantage?

  • New play or old play? Is subsalt lithology truly a new frontier or, as one geoscientist stated, a close cousin to the well-studied shallow sections above the salt structures?

  • Salt-top stacked sections? Does the presence of condensed stacked sections, particularly repeated intervals, above the salt structure generally indicate thicker prospective sections beneath the salt?

  • Subsalt discontinuities? Can a relationship be derived from drilling and analyzing the discontinuities beneath salt structures, or is the prospect of drilling more than one dry hole under the same salt structure too awful for producers to think about?

  • Generations of tectonism? How many generations of salt tectonism are there, and are some better harbingers of hydrocarbons prospectivity than others?

  • Salt shape and remobilization? Does the shape of the salt body or the fact that a salt structure has remobilized at some geological stage serve as an indicator of possible prospectivity?

  • Salt inclusions? Do the inclusions within a salt body, usually at the top of the structure or near the base, provide important information about subsalt formation and contents?

New or old

The subsalt play has opened up at least 40% of the US Gulf of Mexico to producers that prior to 1988 was just not economic to drill? Is it a new play or just some version of an older play that seems new.

The learning curve on discovering subsalt prospectivity is rather flat and stretched out. At this stage of evaluation, a consensus among a small number of geologists, some involved in the play and some not, is that subsalt is a new play economically, but perhaps an old play geologically.

Richard Lindsay of Diamond Geoscience Research posed the question best in a subsalt executive forum in October: "What are the lithologies subsalt and how do the fluids behave? Being typically over-pressured, the elastic properties of subsalt lithologies and fluids'should be' similar to those of the shallow section above salt. So, is subsalt lithology truly a new frontier, or a close cousin to the well-studied shallow section (above the salt body)."

Weighing risks, results

One company that has obtained some of the best results from the subsalt play - two of the five discoveries - is Phillips Petroleum. Phillip's subsalt exploration director James F. Fox, who also addressed the subsalt forum, is cautious in pronouncing any advantage to "creaming off" the best fields and reserves early in the subsalt play.

"The risks often outweigh the advantage of early entry into a play," he stated. "Early entrants usually invest in a play and attendant technologies only once they are proven, and are therefore willing to settle for smaller rewards, but less up-front risks and costs."

One technology useful in the subsalt play - depth imaging - has already been put aside, he said, because it would mature prior to lease expiration. Phillips now uses depth imaging to limit structural risk and to predict pressures and reservoir rock quality prior to drilling, but has moved on to other technologies with which to assess risk and predict hydrocarbon prospectivity.

Reservoir prediction

Invariably, the focus returns to predicting the presence of hydrocarbons before drilling, or at least before the salt structure is penetrated.

Anadarko and Phillips have both focused on establishing some sort of recognition factor by studying the "paleoenvironments of deposition, structural development before and after salt sheet emplacement, and the impact of salt movement upon sediment transport fairways."

In addition to the 33 wells drilled through salt sheets, several hundred wells have been drilled into the tops of salt sheets. These wells, according to Dwight Moore and Stefan Rutkowski of Anadarko, along with Frank Snyder of Phillips, who presented a paper to the same scientific forum in October, yield useful information with which to interpret salt mechanics and subsalt stratigraphy.

In about 40 of the hundreds of wells drilled into salt formation tops, "multiple condensed sections, stacked conformably in thin sequences on top of each other, are encountered in various thicknesses above the salt," the three point out.

"These intervals can be termed as stacked or mega-condensed sections, and their occurrence offers an opportunity to better understand both the suprasalt and subsalt stratigraphy. They are unique and offer an invaluable point of stratigraphic control when encountered."

The authors identify three scenarios for age relationship between supra-salt and subsalt sediments:

  1. Large sedimentary time gaps will often occur along the updip edge of salt sheets where a large listric fault system formed above the salt.

  2. The section can be conformable with little apparent missing section.

  3. A prominent repeated section can occur.

This last scenario - the repeated section - is of primary interest in evaluating subsalt stratigraphy. The three authors point out that, to the extent that wellbore information is made available, wells with condensed repeated supra-salt sections are all showing thick subsalt sedimentary sections with thick reservoir quality sandstone.

But, there is another, related and no less significant, indicator, according to the three authors, that is being used in deepwater subsalt stratigraphy today.

"Thick sands encountered beneath salt are deposited in a lower bathyal (or deeper) paleoenvironment, which appears similar to some recently detailed in significant deepwater Gulf of Mexico discoveries (Mahaffie,1994, McGee, 1994). Sand-rich subsalt sediments can be penetrated after identifying the positions of the updip depocenters and upper slope fairways from where sediments can be transported downdip to lower slope-abyssal depocenters."

Sand-prone sediments gravity deposition, the three point out, bypasses the inflated salt highs and accumulate in the basins surrounding salt scarp edges and fronts.

Other indicators of subsalt prospectivity can be derived indirectly from evaluation of salt structure inclusions retrieved during the drilling or coring, according to research developed by Anadarko and Phillips. Inclusions are materials embedded within salt sheets during periods of salt coalescence and tectonism.

The evaluation of salt inclusions is an extension of studies conducted on onshore salt structures, which can be related to the subsalt stratigraphy. One drawback to these ongoing studies is the wide dispersion of subsalt wells across the Gulf of Mexico.

Other interesting factors - age and form of the salt structure, re-mobilization of sheet salt after initial formation, and determination of the discontinuity environment beneath salt sheets - also could bear some relationship to subsalt hydrocarbon prospectivity.

Slow, costly drilling

The more exciting sector of the subsalt play is in deeper water at the shelf edge or on the continental slope of the US Gulf, generally in 300-3,600 ft water depths. While well logs from nearby wells are fewer in number in deepwater, the possibilities of hitting sizable oil accumulations loom large.

However, the producers in pursuit of subsalt hydrocarbons in these depths are laboring under a heavy burden. Where semisubmersible and drillship drilling units are available to drill wells in deeper water, they are relatively expensive. But, that isn't all.

Subsalt wells require 30-60% more time on the borehole than other types of wells, largely because the wells are much deeper and an extra string of heavy wall casing or liner must be cemented into the well at the salt interval to stiffen the well against salt creep. The average depth of a subsalt well is 16,000-20,000 ft, versus the typical 10,000-12,000 ft wells elsewhere.

As a consequence, subsalt drilling is both cost and equipment constrained. Thus, geologists probably have more time than well data with which to assemble subsalt prospectivity indicators.

On the other hand, subsalt drilling is becoming so standardized that pre-drill planning time has dropped substantially, and so are the number of days required to drill the wells. Mud systems utilizing weighted-up synthetic or salt-saturated water drilling fluids are being used to drill the salt sections. An Anadarko status paper on subsalt drilling stated that salt crystals showing up increasingly in the drill cuttings are an indication of cleanly drilled salt sections.

Even at a brutally slow drilling pace, at least in terms of conscripting a critical mass of information to guide exploration beneath salt structures, producers are putting the bits and pieces of data and possible indicators together. Today, they seem isolated and only vaguely related. Tomorrow, one or more of these indicators may trigger a discovery windfall for the tenacious operator. Then, focused drilling will begin in earnest.

Copyright 1996 Offshore. All Rights Reserved.

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