In the Soviet period (1977 and 1991), two prospecting wells were drilled at licence block 71-1 down to more than 4,000 m deep. In the same period, several other wells were drilled nearby, but they are now on other licence blocks. The two wells in question were strategic prospecting wells, which are designed to explore the local geology, and not primarily to find oil and gas, and all the less to produce hydrocarbons. There was rudimentary seismic data at that time, but without breaking depths deeper than 3,000 m, which generally corresponds to the end of the types of rock from the Jurassic period. The older layers of rocks from the Palaeozoic period (Devonian, limestone), which are estimated to have the greatest prospectivity at this licence area, starts in this area around 3,000 m deep and continues down to just over 4,500 m. The licence area covers a part of the structure in the Jurassic layers, while the remaining part is interpreted from the seismic data going in to the adjacent licence, which is currently owned by Imperial Energy (owned by the Indian state oil company ONGC).
According to current seismic interpretations, both of the wells are estimated to have been drilled in what today seems to be the edge of the structure in question, and thus approximately 4 km from the “centre” of the structure. Once again, please note that there was not any seismic information on the depth at the time of the drilling – the parametric wells were drilled for precisely this purpose, among others – to get information from the depth.
The drilling of Well P-2 commenced in November 1977, but was not completed until September 1979, nearly two years later. The total depth is 4,569 m. Drilling was performed in different sections, upon which each section was logged, and then casing was put in place. Certain core samples were also taken, but with poor quality due to the fissured nature of the limestone. The well experienced an uncontrolled discharge of hydrocarbons due to poor quality of the cementation of the casing in one section of the well. According to the Soviet logs, tests of the hydrocarbons found a mixture of oil, gas condensate and gas.
Well P-4 was drilled in the final stage of the existence of the Soviet Union in January 1991 with deficient financing. P-4 is located 400 m southeast of P-2. The well was designed to be drilled in its first phase to the end of the Jurassic layers, to 2,977 m. After the hole was covered with casing and cemented up to the first target depth, the cement plug at the bottom of the well was to be drilled out before drilling work was to continue down into the Palaeozoic layers. However, a mistake was made during this cement removal, whereby the well was drilled a few metres too deep, down to the beginning of the Palaeozoic layers. Eyewitness point out that the well began to discharge hydrocarbons uncontrollably for days, and was difficult to seal down. This is not found in the log, since it was a mistake that could bring about negative consequences for the people responsible at the drilling site. The only official written indication of this known of it at this time is a telegram (attached below), which was sent on 29 April 1991 from the drilling site to the competent authority where the responsible plastic geologist requests a change to the work programme, since the current programme was not deemed to be “safe”, which was granted.
The telegram text describes in brief that the responsible site geologist requests permission to adjust the agreed work programme by skipping certain loggings in the interval consisting of the upper Palaeozoic types of rock where a gas formation had been encountered. The reason he gave was that a risk situation with potential dangerous consequences could otherwise occur. The response telegram informs the responsible geologist that the sender approves the proposal.
The interval in question was heavily cemented after this and therefore could not be used for controlled flow measurement.
After this incident, the drilling continued and eventually reached the target depth of just over 4,000 m. According to the Soviet logs, a number of intervals were then tested from the bottom and up. These tests never reached the interval described above at the top of the Palaeozoic layers. No more flows were recorded at this depth according to the log as well. BNG’s own geologists suspect today that none at all, or only a few, actual tests were conducted at this depth, because the financing was very deficient at that time when the Soviet Union collapsed. In short, this hypothesis means that the responsible geologists and drilling crew simply logged test results that had not been conducted to avoid problems with incomplete work afterwards. This is however just a hypothesis that will be tested after re-entry into this well is completed. Then we will find out with the help of modern loggings whether the production tests (perforations) were actually conducted in these intervals in the beginning of the 1990s.