Investigation of Secondary Dissolution Pores and its Diagenetic Control on the Reservoir Properties of Gas bearing Neogene Sandstones in the Bengal Basin, Bangladesh

Abstract

Investigation of Secondary Dissolution Pores and its Diagenetic Control on the Reservoir Properties of

Gas bearing Neogene Sandstones in the Bengal Basin, Bangladesh

L. Y. Khondakar, M. B. Imam

The diagenetic history of Neogene reservoir sandstones is investigated based on petrological analyses

of over 16 core samples from Fenchugonj 2 well, an important hydrocarbon reservoir in the North-

Eastern part of Bangladesh ranging in depth from 957-4947m below the surface. The thin section

microscopic technique has been used for this study. Diagenesis in the sandstones primarily involves

the development of types of cement (chlorite, siderite, illite, quartz, calcite etc) and a late dissolution

phase creating secondary porosity. This latter process requires acidic pore waters which are

considered to have been produced during the maturation of organic matter in the adjacent Miocene

source rocks, has created secondary porosity by partially dissolving carbonate cement and some of

the clastic components of the sandstones. In the studied sandstone samples, dissolution of these

leaving an impression of inhomogeneity of packing, elongate pore, oversized pore, corroded grain and

skeletal grain. Our work shows that the remnant of partially dissolved detrital feldspar (skeletal grain)

show 60% to 70% of the individual grain dissolved away and creates secondary porosity. Dissolution

secondary pore is common after a depth of 2190m while it is the dominant type at deepest samples.

We have quantified the thin section porosity of core 2 at depth 1479m is 20% because of low

compaction and absence of cementation. But with increasing depth of burial (increasing compaction

and cementation), core 4 at depth 2150m and core 5 at depth 2768m show porosity 16% and 10%.

Our findings show that core 6 at depth 2768m (gas bearing zone) and core 10 at depth 3615m have

higher porosity because of the higher degree of dissolution and generation of porosity. We also show

that the thin section porosity in core 13 at depth 4248m, core 14 at depth 4540m, core 15 at depth

5721m and core 16 at depth 4947m are 9%, 9%, 12% and 6%. We note that the secondary pores

enhance 50 to 60% of the total porosity of the rock components and improve reservoir quality.

Therefore it is assumed that the Fenchugonj 2 well has a good quality of reservoir sandstones with up

to 20% porosity of the rock components. However, the rapid burial of the sediments in the Bengal

Basin has caused the various diagenetic reactions that take place entirely within the Neogene period.

This study will definitely enhance the understanding of reservoir heterogeneities at the deep burial for

successful hydrocarbon exploitation program.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019