The telltale signs of untapped natural gas in southern Bangladesh

Bangladesh has been lagging behind in conducting petroleum exploration. In the 1960s, the country gained the position of a natural gas province in the world map when Shell Oil Company drilled and successively discovered five very large gas fields in Sylhet and Comilla areas. The discoveries were easy and the explorers used simple rules of petroleum science to bring those wells to success. Unfortunately, Bangladesh could not come out of its "simple way" even after more than half a century since these large discoveries, whereas much of the exploration around the world now engages more complex procedures. The present gas crisis and cries of a depleting gas reserve are a result of the above scenario.

Nevertheless Bangladesh has proved one of its areas, the "Sylhet basin" as it is called geologically, as a very rich gas province. Several very large to giant gas fields have been discovered in close proximity and the success ratio is abnormally high, almost 1:1, meaning success in every one well drilled. The Sylhet basin holds almost 90 percent of the total gas reserve in Bangladesh and includes Bibiyana, Titas, Rashidpur, Habiganj, Kailashtila, Jalallabad and other gas fields. These gas fields are the backbone of the installations supplying gas to the national grid. The question remains: what are the possibilities of other parts of the country having comparable gas reserve?

The simple answer is that the possibilities have not been truly explored. In this respect, the southern part of Bangladesh stands out prominently. Firstly, there has not been enough exploration drilling in the area, meaning that drillings are few and far between in terms of space and time. And secondly, and perhaps more importantly, the wells that were drilled, in many cases, had been declared dry and abandoned although there is clear evidence of gas horizons in the subsurface. The wells were abandoned without complete and conclusive tests following the drilling operations. There are several cases which may be mentioned as examples.

In 1953, a foreign oil company, PPL, drilled an exploration well in Patiya—some 25km southeast of Chittagong city. One observes that there are lots of natural gas seepages in the ground in Patiya hill. The well, Patiya-1, was drilled to a depth of 3100m with strong wireline indication of a gas zone in the subsurface. But the problem in those days was that the drilling used heavy mud fluid which would often bar the gas from coming out because of the pressure of the heavy mud. The well was eventually declared dry and abandoned. This is a clear case which should be retested with modern drilling technology where lightweight mud would allow the gas to rise up.

A similar case was the Jaldi-1 well drilled in 1965 at Jaldi hills about 90km southeast of Chittagong city. The well was drilled to a depth of 2300m. There was clear indirect wireline log indication of the presence of gas in the well but because of heavy mud used in those days, the gas was suppressed from coming up the drill hole.

Geoscientists believe that Patiya and Jaldi are definitely sitting on gas reserves. Bangladesh has been negotiating with Chinese companies to come up with joint venture (JV) agreements with Bapex for several years now to explore Jaldi and Patiya, but no conclusive agreement has been reached. Senior petroleum observers opine that Bapex does not need a partner or JV to drill these two structures and it would be better off to drill these by themselves, thus ensuring 100 percent of the gas for Bangladesh.

A similar structure where gas seepages are abundant is Sitakunda hills. Bapex drilled a well, Sitakund-5, about 50km northwest of Chittagong city in 1989 but could not hit the gas reserve. A reinterpretation of the structure pointedly suggested that the drilling point was wrongly placed in the crestal area where strong tectonic faults badly damaged the structure. Geologists are very confident that should a drilling point be located in the northern part (plunge of the structure) where fault disturbances are not present, one would definitely hit the gas reserve. The gas reserve underground has been seeping out for hundreds of years and can be tapped where it cannot seep out into the undisturbed area. 

In Rangamati district, Shell Oil Company drilled Sitapahar-1 well in 1988. The well, located on the side of Chittagong-Rangamati road, encountered high pressure with uncontrolled gas flow at shallow depth. The well was not drilled further and was abandoned. But it has hit and proved a gas pool at shallow depth. From gas field experiences, a shallow gas reserve is usually accompanied by further gas pool at a deeper level. It is therefore very likely that Sitapahar targeted a gas field of good size, but was abandoned prematurely.

In 2010, Chevron drilled an exploratory well named Kajol-1 in Patuakhali district as per the PSC in Block 7. The well was drilled to a depth of about 3,500m and eventually declared dry and abandoned. But information from responsible sources suggests that thick gas sand was recorded in the well which the company did not want to explore because it was tight sand. In geological terminology, "tight gas sand" refers to gas bearing a sand reservoir from which gas would not normally flow because of low permeability but will flow if the sand is fractured. Producing gas by fracturing tight gas sand is a common exploration practice around the world. Foreign companies may not find this particular target profitable, but it would certainly make sense for Bangladesh to explore the gas with appropriate technology.

A more recent well, Begumganj-3, drilled by Gazprom in Noakhali district also delineated tight gas sand reservoir at about 3,000m and gas from that zone test flowed but the test was not completed and was not pursued further. There are several other examples where exploratory drillings ended and were abandoned before being conclusively tested.

The recent finding that the Shahbazpur gas field in Bhola Island is much bigger than previously thought and the discovery of a new gas field in the northern part of the same island testify that southern Bangladesh has a very rich natural gas based "petroleum system" because of gas generation and accumulation underground. It is geologically unlikely that a rich "petroleum system" works for a single structure; rather it should provide nutrients (gas) to other structures in the circle of influence. Therefore, chances of many other structures having gas pools in the southern coastal area are high.

The above discussion points to the fact that a good number of exploratory wells in southern Bangladesh recorded presence of gas but were declared dry and abandoned before complete and conclusive testing procedures were performed. These wells should be categorised as "gas wells in waiting" and should be reexamined (or re-drilled) with proper and updated technology to bring the gas to surface.

The slogan that gas resources in Bangladesh will be depleted in the very near future is based on wrong perceptions. Yet this notion has set the stage in the energy market in Bangladesh where expensive imported LNG is seen as a "suitable" alternative to gas. Petroleum geoscientists argue that it would be unusual for the largest delta basin (Bangladesh) in the world, which has already found world-class gas fields in one part (Sylhet basin), to be gas-poor in other parts. The evidence presented in this article shows that there are telltale signs of southern Bangladesh being a gas-rich province. At a time when there is a critical gas crisis, Bangladesh should seriously make efforts to bring gas in these reserves to surface.

Dr Badrul Imam is a professor (retired) of geology and an energy expert.