Growth of siderite around Pyrite Nodule

The rim of siderite around the Pyrite in the picture is very clear. However, this is the first time I found such an unique inclusion within last two years of my experience with various drilling programs.

Well, lets take few steps ahead. Here is a simple Eh-pH diagram prepared using the Geochemist’s workbench. I have used:

• Dissolved Fe= 2 mg/L
• HCO3= 600 mg/L
• SO4=200 mg/L

Eh-pH diagram showing stability fields for Pyrite and Siderite

The diagram shows that under low pH conditions, even a slight oxidizing environment can release ferrous iron in the system defined by the large field for dissolved ferrous iron. Pyrite is stable only under highly reducing conditions.

Many of the mine tailing and overburden materials from various mines and specially “Coal Mines” produce Acid Mine Drainage (AMD). Pyrite oxydation is the main reason behind AMD conditions. During various types of mining, the rocks/overburden are disturbed and thereby oxygen being introduced to the system.

There are too many good resources about Acid Mine Drainage (AMD) and pyrite oxidation online. The basic point is, once you oxidize pyrite (Fe-sulphide) you mobilize iron as Fe+2 which later produce Fe(OH)3 after reacting with water and produce acid (H+). So, the acid causes more weathering while the precipitated iron hydroxide forms “yellow boy”!

Lets start our basic work with the most common Pyrite oxidation reaction:

Pyrite + H2O + 3.5 O2(aq) = Fe++ + 2 SO4– + 2 H+

Log K’s of the reaction:

1. 0 °C: 239.6776
2. 25 °C: 217.4000
3. 60 °C: 190.9581
4. 100 °C: 166.0899

Polynomial fit:

log K = 239.7 - .9517 × T + .002644 × T^2 - 5.354e-6 × T^3 + 4.55e-9 × T^4

Equilibrium equation:

log K = - log a[H2O] + log a[Fe++] + 2 × log a[SO4--]

- 3.5 × log a[O2(aq)] + 2 × log a[H+]

Pyrite can form siderite under high alkalinity conditions:

Pyrite + H2O + 3.5 O2(aq) + HCO3- = Siderite + 2 SO4– + 3 H+

Log K’s:

1. 0 °C: 239.4235
2. 25 °C: 217.6214
3. 60 °C: 191.8154
4. 100 °C: 167.5989

Polynomial fit:

log K = 239.4 - .9323 × T + .002634 × T^2 - 5.471e-6 × T^3 + 4.962e-9 × T^4

Equilibrium equation:

log K = - log a[H2O] + 2 × log a[SO4--] - 3.5 × log a[O2(aq)]

- log a[HCO3-] + 3 × log a[H+]

In the picture we see that the formation of the siderite rim actually stopped the oxydation of pyrite and preserved it inside.

If you have any other theory about the formation of siderite and relationship to the pyrite, let me know. I am open to all suggestions!

Drill string Packer Testing is one of the simple test that is widely used for hydrologic investigations. There are three main types of Packer testing - Wireline Packer tests, Drill String Packer Tests and Falling Head Packer Tests. Three types of packer tests vary in the configuration of Packer assembly. Just go to the end of this post and check resources for detailed discussion on Packer Tests.

Now let me share some of the photographs I took during the test!

Packer-Test-1-inch-Drill-Pipe

N2 Gas Cylinder

Various Airlines

N2-Gas and Airline Connections

Tape used for Depth Measurements

Various Types of Packers

Things we used for the packer test:

1. N2 Gas
2. Inflatable Pckers
3. 1″ waterpipe / A-Rod
4. Perforated Pipe
5. Airline
6. Gauges
7. Flowmeter

Tips that I use:

• Define Packer Test zones based on the interval between the packers. I recommend a 10 feet packer interval for easier calculations.
• Make sure you know how to read the flow meter. Varius rigs have various set ups.
• Take professional grade long tape (the maximum lengh available in market is 300 feet). If your test hole is less than 300 feet, you are in luck. use just one tape, attach the bottom of the tape to the lower end of “zone of testting”.
• Check for any leaks before each packer test, check joints on the airline, sometimes you can hear the sound of the leak or feel the air flow in your hand.
• Run water for couple of minutes before each test to remove air bubbles from the system.
• Always recored the stick ups above the ground for each test.
• Apply enough pressure to the packers to seal them properly to the wall of the borehole. If not sealed properly, the water pressure during the test would eject the packer uphole.
• As you go deeper, you need more pressure for the packers to seal properly. It is recommended that you read the namual from the manufacturer. At least 220-250 psi N2 pressure is needed for all packers.
• It is good to have some formation that takes no water. Flowmeter does not move at all in those horizons. This proves that the packers are sealed well and there are no leaks in the system.
• Some formations are so fractured that you do not need almost any water pressure. They will consume water very fast. You will have hard time to produce any amount of water pressure for such formations.
• For general shale or sandstone, keep the water pressure around 100 psi. (Even 40-60 is good enough for may formations). Always check water pressure before taking any measurements. Sometimes the water pressure drops over time.
• Do not close the valve at the water supply tank before turning off the pump at the drill rig.
• Make educated guess. Always.

1. Standard Operating Procedure for Packer testing by Michael Royle
2. Packer Test - Short Description by USGS
3. Inlatable Packer Application by Clem Rowe

Strip Mine or The Heaven

Coal-Hauling-Truck-In-Stripmine

CATs-in-action

Stockpile Generation

Just some amaizing pictures I took!

It is very important to know the extent of the weathering zone for coal mining purposes be it a surface mine or a deep mine. Weathering zones are often identified by brown sandstone and they usualy contain many high angle fractures. The zone becoming impompetent compared to rock units adjuscent to it and may create potential mining hazards if overlooked.

Weathering Zone Defined by Brown Sandstone

Concept of the Weathering Profile

Coal sampling methods

The better is the quality, higher is the price of a coal. Coal sampling and analysis determine whether the coal can be sold as a cocking coal, metallurgical coal, prime coal or lower grade coal. Many times the customers specify a particular quality limit for buying coal. Coal sampling and analysis is necessary to check for the quality of the coal seams to be mined or under production.

Sampling of coal is very important for a coal reserve evaluation. Sampling is required for various reasons such as:

1. Part of mine development
2. Part of reserve evaluation and delineation of the mine projections.
3. Part of quality check of the coal seam.
4. Also required as a part of mine permit application.
5. Required for SEC filing in the USA.
   

1. In-situ samples: Samples from coal outcrop, exposed seam in opencast or deep mine, cores or chips from drill holes.
   • Grab Samples: Often the coal sample is not representative of the entire coal seam. One of the least reliable coal sampling method.
   • Channel Samples: One of the best sampling methods for coal. When the coal sample is collected from an outcrop, the exposed area should be cleaned to avoid the weathered exposed coal surface. Normally a small is box cut is made at the coal outcrop exposing the entire thickness of the coal seam. For a relatively thin seam only one coal section is recommended. However, if the seam is really thick, two or more coal sections may be necessary to sample entire seam. The channel sample should be cut perpendicular to the bedding plane and stored in plastic bags. The sampling of the full seam provide overall quality of the seam including all boney and mineral matters within coal. Sometimes a more detailed analysis is required where the coal seams are collected separately from bone and rock partings. However the bone and rock partings are collected separately. Then the position and thickness of the coal - bone - rock samples are recorded and sent to the lab for detailed analysis. This type of channel sampling is knows as Channel Ply sampling. At many times during ply sampling a small portion of the roof and floor rock of the coal seam are also included in the coal analysis to allow “out-of-seam-dilution“.

• • Pillar Samples: In some special occasions the “strength” of the coal becomes important mainly in the underground mining. A large block of undisturbed coal is usually samples carefully from some specific areas of potential problems or areas with known problems. Sampling scheme is similar to Channel sampling.
  • Core samples: Core sampling is mainly a  part of the exploration and reserve evaluation stage. This is however very important for the development of a future mine. A geologist is usually assigned to supervise a drilling program. Coal samples are collected in wooden boxed carefully in the field if not sampled at the field. Most of the time an e-log is prepared for each completed hole in recent time. A geologist check the e-log for the coal thickness and adjust the “core recovery” for the collected coal seam. Core recovery is usually >90% for the coal seams however a poor core recovery is possible if the driller is not very much experienced with coal drilling. Sampling method is similar to Channel Ply sampling. On the logging sheet, the depths of each ply sample intervals are recorded before sending to the lab. Total seam, Total coal and the % of coal recovery are also recorded on the sampling tags as a note to the coal analytical lab.

Ply Sampling Method

Coal Sampling Technique

Coal Sampling Technique

• • Cutting/Chip samples: This is a much less accurate sampling scheme than the core sampling. Cuttings are generated by rotary type drilling where no core is recovered except chips. Air flush or mud-flush rotary drilling is a much faster drilling and mostly used for gas wells. This king of sampling can only give us a very general analysis of the coal. It is very difficult to collect samples and most of the time we have lots of impurities mixed in it. Also, the exact depth of coal can not be accurately recorded unless generated from a geophysical log after drilling is completed.
  • Bulk Samples: Bulk samples are collected mainly for larger scale tests, to check swelling properties of various coal seams, to rank coal by High Pressure coals and low pressure coals and so on.

1. Non in-situ: Sample from coal stockpile, coal transport containers etc. They are often not representative of the coal seam that is mined. They are also may be diluted with “out-of-seam” products from the roof and floor strata. Sometime the stockpile contain a  mixture of two or more seams to meet certain quality standard specified by the client. Very rarely geologists are asked to perform this type of sampling.

Reference: For more detailed coal analysis technique please refer to “Handbook of Practical Coal Geology” by Larry Thomas.