"This is an important step for our operations at the El Bagre mine, giving access to an additional 65,000 ounces of proven and probable reserves at a substantially higher grade than we are presently mining," president and CEO Javier Cordova said.
The portal was expected to cost US$750,000.
The company, previously Para Resources, had acquired El Bagre next to its El Limon mine earlier this year.
Cordova said the tonnage available at Cordero Balvina would "complement and eventually supplant" current tonnage being mined at El Bagre's Mangos and La Ye underground mines.
"Total production at El Bagre in 2021 from all portals is expected to be 140,000t at an average grade of 6g/t," he said.
In June, La Ye was targeted by armed robbers who stole 475oz of gold equivalent in doré, after taking advantage of a change in protocols during the transition.
Soma also said it had produced and sold 655.88oz Au-eq of doré last week at El Bagre for net proceeds of $1.28 million.
The company had produced and sold a total of 1,204.33oz Au-eq in doré in July for $2.2 million.
Cordova recently told Mining Journal the company was targeting 40,000oz of annual production in 2022.
Soma closed an oversubscribed placement in July which had raised C$4.44 million at 15c per unit.
Its shares (TSXV: SOMA) last traded at 67c, capitalising it about $40 million (US$30 million).