Unconventional Plays Revealing New Stories of the Earth

Convection vs Conduction in Thermal Maturation of Organic Matter for the Generation of Hydrocarbons

By Waqas Haider*

Surprise...!!! Hot Spots (Hydro-thermal Fluids Generation Points) Can be Useful in Unconventional Plays

I was caught by a surprise while coming across this study, it looked strange to me because we've been studying so far that radioactive elements in the Earth's crust emit enormous heat, and that heat reaches the Rock Columns gradually (not simultaneously) and cause organic matter in the rocks thereby to get thermally mature which is necessary for the generation of hydrocarbons. But here is a different story, the evidences has been collected by a member of AAPG in First Shot Field (Eagle Ford -USA) as well as Stanley and Parshall Fields in Williston Basin (USA-Canada). The whole new story is as follows:

In the beginning while evaluating the unconventional plays (new phenomenal shale play bonanza), general perception was that the rocks were homogeneous throughout area of interest. But then in Oil & Gas industry the E & P (Exploration and Production) players realize via drilling bit that heterogeneity rules, and homogeneity and isotropy are not even bit players in the big picture. In reality these dense rocks can considerably vary from one well to an adjacent well adding a considerable curiosity and associated risk in leasing and drilling. 

Better Handling of Potential Area (Before we go for Seismic Technique):

Seismic data techniques are utilized to analyze and to evaluate the production potential of a prospective area. But Seismic Method is an expensive method so it is recommended to go for applying small cost techniques to grasp on with which to proceed. These methods include remote sensing, integration of inorganic and organic petrography (well data), gravity and magnetic methods and other various types of thermal maturity data (basin studies) to recognize the areas having the potential to give us commercial production. 

Once the Geoscientists get better understanding of a certain potential area, they can then opt more comprehensive and expensive technologies like seismic for further "better handling of the prospective area". 

Fig. 1: Conduction and Convection (with Faults and Fractures
acting as Conduits for Hydrothermal Fluids). (Credits: AAPG).

Heat Source:

You're thinking it would be conduction ... Umm No..!! It's Convection Actually..

According to Miss Edman (a member of AAPG): "experts have independently come together on the concept of using a combination of less expensive screening technologies to identify areas of localized high heat flow where recurrent movement of basement faults in areas already known to contain rich source rocks results in the maturation of hydrocarbons by Hydrothermal Fluids". 

"The published findings that at both a mega- and a micro-scale, an internally consistent genetic model can be developed, showing in multiple diverse locations that unconventional play sweet spots are often related to hot spots".

The "General Model of Generating Hydrocarbons" has been compared with "New Model of Generating Hydrocarbons (in which convection plays the key role)". In general model of generating hydrocarbons where we have organic matter in the source rocks and conductive heat coming up from the basement into the sedimentary section . That heat matures the organic matter causing hydrocarbon generation. 

In Earth's crust there are radioactive elements emitting heat that comes up by the conduction into the sedimentary section, and that's the source of heat for most hydrocarbon models, or basin models based on burial history.

But in the work by Miss. Edman, she said: "we actually have fluid movement, and heat from the hot fluids is causing maturation of organic matter, so it's a different heat source".

Convective Heat from the Fluids is causing the Generation."

DEM (Digital Elevation Model) of North Dakota.
(Credits: AAPG)

Convection vs Conduction:

The two types of heat transfer are quite different:

Conduction:

With conduction, the transfer of heat occurs quite slowly from the bottom to top of successive rock units that are in direct physical contact, according to Edman. 

Convection:

With convection, there is a rapid elevation of temperatures in multiple rock units simultaneously due to the relatively unconstrained movement of hot fluids (hydrothermal fluids). 

Experts emphasize that convective heat flow via hydrothermal fluids is much more efficient than the transfer of heat by the conductive heat flow. Also igneous activity in the shallow crust is more common than people realize, it;s this igneous driver that's the ultimate origin of the hydrothermal fluids. 

Faults Acting as Conduits for Hydrothermal Fluids:

The flow of hydrothermal fluids into the sedimentary section can be attributed to conduits provided by recurrent movement on faults and lineaments that extent to the basement.

Two Major Indicators of Sweet Spots in Unconventional Plays:

Two main elements needed to find the hot spots leading to the prediction of potential sweet spots are:

1. An igneous driver (igneous activity) for the hydrothermal fluids' generation 
2. A system of naturally occurring faults and fractures acting as conduits for hydrothermal fluids. 

Evidences from the Fields:

Besides serving as conduits for hot fluid flow, natural fractures are important to create areas of increased permeability. Application of combination of techniques are of key importance and they work best in a particular area of interest. 

For example in area of dense vegetation GPR (Ground Penetrating Radar) and Remote Sensing are not the ways to go. Also a well or more than one wells can help us in the integration of organic and inorganic petrography. 

A micro-photograph (Edman showed) that shows carbonate cement that came in with hydrothermal fluids. Then you have these trails of oil, fluid inclusions included in that carbonate cement showing that you had generation of that oil at the time the hydrothermal fluids moved in.

Oil inclusion trails in carbonate micro-veins of the upper Bakken
Member in the Long 1-01 H Well - microscal evidence that hydro-
-carbons were generate in situ at Parshall Field.  

It has been noted that AAPG member Dan Jarvie and his colleagues demonstrated in 2011 that oil at Parshall Field (Williston Basin) in North Dakota was generated in situ. 

That's what this story tells: "A lot of people think the hydrocarbons migrated in from the west where the Bakken is more thermally mature, but looking at the biomarkers from Parshall Field, they're not all that mature. 

Edman and her colleagues have some convincing examples from the Eagle Ford at First Shot Field in Texas and the Parshall and Stanley fields in the Williston Basin showing that better production is related to areas of localized convective heat flow. 

This is a great inexpensive way to search for sweet spots in these Unconventional Plays. 

References:

       American Association of Petroleum Geologists. 

Louise S. Durham 

Janell Edman (Principal at Edman Geochemical Consulting in Denver). 

*
Waqas Haider
Student of M.Phil. Geophysics
Department of Earth Sciences
Quaid-I-Azam University Islamabad (45320) | Pakistan. 

email: geomindx@gmail.com

cell: +923215140154